It is possible to use your own tables with ASP.NET Membership because it uses a provider model. However, this is outside the scope of DevForce and it is not encouraged to do so. You can see the website below for more info.
A few suggestions:
1. You can issue a Forms authentication ticket (cookie) when a login occurs and not use the whole ASP.NET Membership architecture. To do this, you could sub-type the AspNetAuthenticatingLoginManager and override the ValidateUserCore method to do your own validation against your own user table.
2. Or you can implement your own login manager entirely and just call FormsAuthentication.SetAuthCookie to set the cookie.
Hope this helps.