In reply to romanzy.

One way you could go about this would be to use a plugin like Gravity Forms and use the User Registration add on. There you could use a registration form to handle registering multiple different user roles.

Honestly, though, user roles is really more of a band aid. Using Member Types would be a better solution and conditional Profile Fields by Member Type will probably be a part of BuddyPress 2.4 (see the trac ticket mentioned by Boone above).