You can setup your add to cart experience however you would like it to be. If you would like to add the item to the cart but not take the user to a page you can keep them on the same page. Your show region could just be a check for items in the cart or a particular item in the cart. This part would need to be custom coded though. You could then show a button to register or view the cart or whatever you would like.
If you have the ability to know when the registration has been filed out then you can just display a button on the cart page to register if it has not been completed already. You would also need a check before the user checks out to determine if the registation has not occurred, if it has not then you would need to prompt them to register before finishing the checkout.
