Re : adding fields to register.tpl/php
1) you can use the user defined fields.
http://tikiwiki.org/tiki-index.php?page=UserPreferencesAdmin
2) in 1.9 you can use a tracker too. I have no experience about that. But I know mose uses this functionality. The login page uses the plugin tracker. The registered group uses a tracker (admin -> groups -> edit registered -> tracker) ...
Good luck
sylvie