There isn't, quite yet. The ability to have additional fields during registration is a part of JFBConnect 3.2, due for release next week. If you'd like to see our sneak peek of that release (including this feature), check out:
www.sourcecoast.com/blog/item/106-jfbconnect-32-sneak-peek
Hope that helps, and let us know if you have any other questions!
jomsocial - force fields complete after facebook login
