If you're going to store them in the DB then Customizing Auth is easiest: http://web2py.com/book/default/chapter/08#Customizing-Auth
Subclassing is going to be more difficult.
In a model file that comes before "auth =" do this:
from gluon.tools import Auth
class MyAuth(Auth):
def navbar(self, prefix='Welcome', action=None):
""" subclassing navbar() """
...
Then switch "auth = Auth(globals(),db)" to "auth = MyAuth(globals(),db)"

