On the 9Lessons site there's a recent tutorial showing you how to make a customized signup form that still uses the Facebook authentication as its base.

Using facebook registration plug-in users can easily sign up on your website using facebook data. With one simple facebook login the form will be filled with user appropriate data. This plug-in is a simple iframe you can place it anywhere on your webpage. You can also add a custom field if facebook doesn't have.

He walks you through the four (or five, depending on your view) different steps you'll need to make the simple form. You'll need to have a database to store the user's local data in, including their Facebook uid. A simple iframe on your page gives you a Facebook login form and a basic callback page handles the successful submission and saving of Facebook's response. You can add a custom field in the form's json and it'll pass right on through too.