Hi,
SiteUser.Login is just a boolean function for validating login credentials so looking up valid credentials and passing them to that function is useless.
Logging in is a matter of setting a cookie. You should google for FormsAuthentication.SetAuthCookie and FormsAuthentication.RedirectFromLogin to learn how to set a login cookie.
However, what you are doing sounds like pretty much bypassing any real security and making it completely insecure with only some illusion of security. If you create a page where all one has to do is pass in a username it it logs you in as that user that means any user who knows about it can pass in someone else's username and login as that user without knowing the password. Sounds like a very bad plan to me.
Joe