Assign custom username & password, automatically during registration.

Hi, I have assigned auto increament userid (say, E-101, E-102.....) to username field of a membership wizard. its showing the text but the username is treating the field as empty until i change anything to the field.
also can anyone help me  to assign a autonumber for password (say, 123456) during registration process. I am using .net 3.5.
...