Chat transcripts are saved under application support, in these weird files for each contact that only Adium can read. If you want to directly copy it you can view it by right clicking on a contact (on your buddy list) and hitting "Chat Transcripts", then you can copy all the conversations manually to a txt file if you want. Once in the browser you can view all your contacts conversations (not just the one you selected) and all the different dates.
Hm thas really bad... I dont want to copy every day each of them you know...
thats a lot of work