This is a feature, along with a few others, that we're planning to add to the Community Builder and JomSocial plugins soon. This feature would only work when user creation is set to "Full Joomla User", since that's where we can ask other questions of the user. Right now, our focus is on some finalization of the Joomla 1.6 version of JFBConnect, and after that, we'll be polishing up some features that have been requested, like this one.
Hope that helps answer your question!