Chinese speakers have phonetically transcribed the word "geek" as jíkè 极客, qíkè 奇客, etc., and these transcriptions are fairly widely used and recognized, even among Mandarin speakers (the initials would be velars in many non-Mandarin topolects, so they would sound more like "geek" than do the Mandarin pronunciations). So far, I don't know of any Chinese character transcription for "nerd", certainly none that is broadly circulating.
Although the English word "nerd" is widely known and used in China, it is puzzling that no character transcription has become popular for it, as have jíkè 极客 and qíkè 奇客 for "geek". One of my hypotheses for the reluctance of Chinese speakers to attempt a transcription of "nerd" is that the final consonant cluster would be very difficult to render or approximate with characters. But that could just be one of the reasons Chinese have not attempted to transcribe or tanscribe-translate "nerd". It could also, or simply, be that Chinese have a harder time understanding the meaning of "nerd" than they do the meaning of "geek".
To force the issue, I asked about a dozen native speakers how they would transcribe "nerd" with Chinese characters. Here are the results (thanks to Gianni Wan, Cheng Fangyi, and Cao Lin for help in assembling this information; note that I usually only give Modern Standard Mandarin [MSM] pronunciations; in most cases the characters are meant merely to transcribe the sounds of the English word, but sometimes they also secondarily convey a meaning that is more or less appropriate, so I also give literal translations of the individual characters):
From a Jin 晋 topolect speaker (age 30) who comes from Yulin, Shaanxi, but is living in Canton: nè 讷 ("slow of speech; mumble; stammer") or nèdé 讷德 ("slow-of-speech virtue")
From a Hakka speaker (age 17) in Meizhou, Guangdong Province: nède 讷的 ("one who is slow of speech")
A second reply from the same Hakka speaker: nèdāi 讷呆 ("slow-of-speech dolt"), though he admits that the sound correspondence is not as good as his previous suggestion (I've also had this transcription confirmed from other sources)
From a Cantonese speaker (age 31) who comes from Liuzhou, Guangxi, but is living in Shenzhen: nàdé 纳德 ("accept / admit / pay / offer virtue"), though this transcription is also used for "Nader", as in Ralph Nader (I've also had this transcription of "nerd" confirmed from other sources)
From a Mandarin-speaking graduate student (mid-20s) at Tsinghua University in Beijing who comes from Hunan: nèdòu 讷豆 ("slow-of-speech bean")
From a speaker of Cantonese and Shanghainese, who comes from the north but is living in Shanghai: nèdá 讷答 ("slow-of-speech answer")
From a speaker of Cantonese who comes from Xi'an, but is living in Shenzhen: nède 讷的 ("one who is slow of speech")
And the last one, from a speaker of Shanghainese, who both comes from and is now living in Beijing: nèdé 讷德 ("slow-of-speech virtue")
It is interesting that, in a separate note, this individual explicitly connects the nè 讷 ("slow of speech") of his ad hoc transcription nèdé 讷德 ("slow-of-speech virtue") with the bookish term mùnè 木讷 ("stiff; dull; sincere and honest but slow of speech"). I also find it fascinating that so many of my informants independently hit upon that rather odd character nè 讷 ("slow of speech; mumble; stammer") to render the "ne-" portion of "nerd". In truth, however, there are very few characters in Mandarin that are pronounced "ne", though characters pronounced "na", of which there are many, could also have been used. No matter what, it is serendipitously appropriate that nè 讷 ("slow of speech; mumble; stammer") conveys a meaning that Chinese speakers associate with "nerd".
In my original post on the subject of "nerd" in China, I noted with some puzzlement that èr 二 ("two") had been mentioned by several informants as recently being used in contexts that resemble those in which "nerd" appears in English. In my latest round of inquiries, it was called to my attention that a fuller form of the expression is èrdāi 二呆 ("two / second dolt"). This still didn't make sense to me until I recalled that that the pronunciation of èr 二 ("two") in Japanese is "ni", and it also has an "n-" initial in older forms of Chinese. In Cantonese èr 二 ("two") is ji6, so that wouldn't work, and in Taiwanese we have li7 / ji2, but in Shanghainese we have ni2, in Hakka we have [海陆丰腔] ngi6 [客语拼音字汇] ngi4 [沙头角腔] gni5 [陆丰腔] gni6 [梅县腔] ngi5 ng5 [台湾四县腔] ngi5 [客英字典] ngi5 [宝安腔] ngi5 [东莞腔] ngi5, and in Chaozhou (Chiuchow, Teochew) we have no6 (nõⁿ ) ri7 今又音ri6. Consequently, in many Sinitic topolects, 二呆 ("two / second dolt") would sound something like "nidai", which is roughly within the ballpark for "nerd".
To return to the more direct character transcriptions of "nerd" that I discussed above, I think that the ones which incorporate 呆 are particularly clever, since that both sounds a bit like the final consonant of "nerd" and conveys the sense of "silly, daffy; dullard; dolt" — although that doesn't mean the same thing as "nerd", it is close to it in the eyes of many Chinese. Meanwhile, again in the eyes of Chinese speakers, nè 讷 ("slow of speech; mumble; stammer") does fairly well for the "ne-" part of "nerd". Consequently, I vote for nèdāi 讷呆 ("slow-of-speech dolt") as perhaps the best current Chinese transcription of "nerd", though, of course, it doesn't really sound a lot like "nerd", nor does it mean exactly what "nerd" does. In the Chinese linguistic and social context, however, I suppose that something like nèdāi 讷呆 ("slow-of-speech dolt") will have to do, unless some daring folks simply want to adopt the word "nerd" itself and borrow it into their language.
As for the "r" sound of "nerd", it seems that it has been thrown to the winds.
Just after I finished the draft of this post, I received from one of our senior lecturers in Chinese some additional observations on the matter of "nerd" and "geek". She suggested that "nerd" could be transcribed as nèdēer 讷嘚儿 and said that she had actually encountered this transcription online. We've seen the nè 讷 ("slow of speech") character for the first syllable in many of the above listed transcriptions, but dē 嘚 offers a new twist. On the one hand, because of the mouth radical, we may think of it as just a pure sound, but on the other hand it has a topolectal meaning which may have been subliminally present in the mind of the person who chose it, viz., "chatter" (the more usual meaning of "clatter of a horse's hooves" is irrelevant). The er 儿 sound is also interesting, since — although it comes at the end — the person who selected it may have intended to represent the medial "r" of the English word. But the final er 儿 also introduces an entirely new aspect to the conversation on "nerd", it conveys the sense of "young man; fellow; boy", thus raising the issue of gender, inasmuch as er 儿 tends strongly (while not exclusively) to refer to males. In English, it seems to me that girls can definitely be nerds. I'm certain that I've heard people talk about "nerdy girls".
Other renderings of "nerd" that our senior lecturer told me she had seen online are nèkè 讷客 ("slow-to-speak guest"), with the kè 客 ("guest") perhaps invoking something of the notion of xiákè 侠客 ("knight errant") (it clearly is not serving any transcriptional purpose here), and nèdēzú 讷嘚族 ("clan / tribe / family / race of nerds" [nèdēs, i.e., mumbling chatterers]).
The gendered conception of "geek" is clearly shown by our senior lecturer's coinage, jípǐnnán 极品男 ("top grade / quality / class / rank male"), which also shows that geeks to her are by no means situated on a low rung of the social scale.
Finally, our senior lecturer told me that her favorite proposal for "nerd" in Chinese is "BBT男", where "BBT" stands for "The Big Bang Theory", the title of a television series, and nán 男, of course, means "man; male; guy". So "BBT男", i.e., "nerds" to our senior lecturer, are the kind of guys who elaborated the Big Bang Theory!