If I'm understanding your correctly, you have a preexisting table called 'users'. If this is the case then you're running up against the fact that, by default, web2py expects to handle your database for you (defining relations, properties, etc).
One fix, if you know the layout of your database is to first define your models in web2py to match exactly what is in your tables; run it once to generate the sql.log; modify your models; run it again.
Another is to turn off migration so that it doesn't try to modify the database structure. This means that you loose the wonderfully useful auto-migrations and have to do the ALTER statements yourself.
Which ever you choose I'd strongly recommend that you first take a quick read through of the Migrations section of Chapter 6 of the web2py manual: http://web2py.com/books/default/chapter/29/06/the-database-abstraction-layer#Migrations
I hope that helps