You can retrieve the userRole you want from the database. In the Xpath you can use something like [id = '[%UserRole_User%]'], where User is the name of your role.
Then you can do a change of your account and set the association UserRoles with the role you just retrieved.
Hope this helps!
Given the UX you are going for, I recommend you check out the AutoLogin module. You have to establish the user account and change from Anonymous to a User Role, so the AutoLogin module is what you are looking for. Otherwise you just are assigning the role but then they get kicked back to the login page to authenticate.
Is this what you meant @Johan?