Server variables are probably best for this. When the user registers you can add in some set session value server behaviors to the top of this page and trigger on the submit button being pressed. You would want one variable for each value you want to prepopulate in the login form. On your login page you can just bind the session variables to the form fields through the bindings tab.
There are other ways you can do this though. In your access checks to determine if a user is logged in what is the criteria? It is very common to store the user's id in a session variable called userID. Depending on how you determine if a user is logged in you could just auto log them in after they register by setting the userID session variable or whatever you use to determine this.