Pro Sites level at sign-up + check up

Hi WPMU,
I was wondering if it is possible for a user to select a pro sites level at registration and be taken to the checkout page before completing sign up. Within the current functionality, I understand that a user must first create an account/blog then they would be able to click on the upgrade button to then upgrade. We would prefer our users to see this option right away when arriving at our registration page, frizzybiz.com/register.

Thanks for your advice and help.