A function like this does exist if you are posting data back using the POST method, but not for passing the products in the URL, however, anything is possible, but some things take a little work....
With this problem, you would need to add some custom code to the includes/extra_cart_actions/ folder. In here, I would add a file which checks for a GET action of "add_multi_get_products".
You will then need to go through and iterate through the product ids, and quantities, and use something like:
Code:
$_SESSION['cart']->add_cart($products_id, $_SESSION['cart']->get_quantity($_GET['products_id'])+$qty);
You will need to include a little error trapping, for circumstances such as the customer is trying to order more than allowed etc.
Absolute