I can think of a couple of ways you could handle some things like this. The first part of this is to make it so that if you add the item to the cart a message displays letting the user know that the item has been added to the cart, there will also be a button to allow the user to proceed to the cart.
To do this you should set the redirect page in your add to cart server behavior to go back to this same page with a URL parameter present. So the end of the redirect might have this in it:
?added=1
Then you would add in your message and the button where you would like the user to see it after adding the item to the cart. Once you have the message and the button there you would add in an if statement around this part so that it is only displayed if the added url parameter is set to one. Here is what that if statement would look like:
<?php if(isset($_GET['added']) && $_GET['added'] == 1){ ?>
your message and button code go here
<?php } ?>
Once this part is done you can move onto the next part. So when the user clicks on the button to go to the cart you would like them to fill in the registration information first correct? If this is the case then you will send the user to the cart page. Then on the cart page you will check to see if they have filled in this information. If they have not they will be redirected to the page that will gather this info. If they have filled in the information then they can proceed to the cart and checkout.
To do this your registration form page will need to store the information in your db, then set a session variable. Then on the cart page you just check this session variable so see if it has been set to the predetermined value. If it has then the user will stay on the cart page. If this session variable does not exist or does not have the correct value then you would have a header call that would redirect the user to the registration page.
The set session value on the registration page should set a session variable to a particular value when the information is inserted into the db.
On the cart page you would add in an if statement to check on this session variable, if it is not set correctly you would redirect them to the registration page. Here is an example of what that code would look like.
<?php
if(!session_id()) session_start();
if(!(isset($_SESSION['registered'])) || $_SESSION['registered'] != 1){
header("Location: registration.php");
}
?>
