Well this depends on what registration process you use on your site and what fields you want to add, because if you use the registration to the forum (phpbb) the fields are fairly easy toi custimize, just do a search on phpbb.com and see for your self. Also i found a mod for this located here http://www.phpbbhacks.com/viewhack.php?id=236 but you have to know something about templates and php.