You should talk to porkupan (aka boroda) or igorsk on this forum. Or search for porkupan's posts here. Better yet, go to
http://www.the-ebook.org/forum/ for complete instructions if you read Russian.
They are the ones who have created the method for putting Cyrillic support (and other languages) on Sony Readers.
I've done it myself following porkupan's instructions in Russian. However, I think he also translated them into English.
By the way, I need to point out that the method is complicated and can kill the reader for good because it modifies the firmware. So, if you are not comfortable with the risk, you should probably go ahead and buy that reader on ebay.