The best option would be to create a user_profile and have the user
has_one :profile, :class_name => 'user_profile'.
If you have to have it in the user class you have to add them here:
attr_accessible :login, :email, :password, :password_confirmation, :security_question, :extra_fields
Thats what I'm guessing but I guess it depends on the error...
Did you add the extra fields using migration?