[web2py] user table just minimal customization

[Manuele Pesenti]

is there a way to add just a single field to the user table without 
re-wrintig a customized version as explained in

http://www.web2py.com/book/default/chapter/08#Customizing-Auth
I mean... if I only want to add "username" field in order to use it in 
place of email during user login... how can I do it? if I can...

thanks

        Manuele