Hello,

You can customize the checkout process as it is demonstrated in the following section of Developer's guide: Developing custom dialogs for checkout

The Shopping card control provides a similar functionality as the wizard control, however, it was not designed for user's registration. I would go for creating a custom web part based on standard either Wizard control or Create User Wizard control (Create User Wizard), as you would need to completely modify the Shopping card functionality.

Best regards,
Michal Legen