You might be able to, but I think it would be easier to roll your own
solution, since much of the controller logic deals with validating and
shuffling data around after validation. There's also the 'lost
password' functionality, which relies on email.
To make your own system, I think all you would need was a controller
and form that saves directly to the tg_users table. You could probably
borrow the forms from registration/widgets.py (NewUserSchema) if you
wanted to save a little work. You might also want a 'edit user' type
form, but again you could reuse the registration widgets as a base
(ExistingUserSchema).
Where are you planning on hosting this app? It's a little surprising
to me that your ISP doesn't offer outgoing smtp.