If you look into common.py you find this...
auth = Auth(session, db, define_tables=False) # define the object
db.define_table('auth_user', ....)
auth.enable(uses=(session, T, db), env=dict(T=T)) # exposes the action
and you can make your own auth_user table but make sure it contains the basic fields.
There is only one table auth_user.
You can also do:
auth = Auth(session, db, define_tables=True, extra_fields = [Field('birthday', 'datetime')])
auth.enable(uses=(session, T, db), env=dict(T=T)) # exposes the action
and just add fields to the table.