Built-in webapp means it is not django project? Then you should write an
authentication backend:
http://docs.djangoproject.com/en/dev/topics/auth/#other-authentication-sources
And then... uhm, you should either replace
django.contrib.auth.models.User model with something which point to your
user database (just with monkey-patching) or maybe creation of User in
your backend. Not sure what will be better in your situation.
--
Alexander