Okay, just re-reading this again. I don’t think you should use bp_core_signup_user().
For your custom script, you can try using wp_insert_user() and then add in the extra profile information later.
Or, you can try using the Import From Ning plugin, which also works with CSVs:
http://teleogistic.net/code/wordpresswordpress-mu/import-from-ning/ (read the comment section)