There was a topic about that, but it's not here any more, maybe lost during import. Rick said the Bio field was discarded because there is a bigger change coming up for the user profiles.
Anyway, I also wanted to keep the biographies. What I did is to use the USER_EXTRA_FIELD_1 for the Bio during the import. This field was used for ICQ before, so it's normally unused after the import. If you already did the import it's too late, but if not is't just an easy modification to threads_import.php: