Custom Login Screen

I wanna know if i can develop a login screen diferent to the NW login screen. I mean that i want to create a view in WebDynpro that receives some parameters from user and execute the login cheking against the UME of Was.

If so, i wanna know how to check that user parameters are valid or not to login in the app. I mean how to pass that parameters to the UME.

Thx and Rgds

Gregory.