One way you could do this would be to put an add to cart server behavior on a page that you know the user will go to regardless, like the product listing page or the cart.
The idea is that you have this add to cart server behavior on the page and the trigger is based on a session variable being set. Since this session variable will not be set by default the add to cart will run and add the item. You then edit this server behavior on the page to add in the code to set the session variable. This way after the code has run the item will be added but not again.
There are other ways you can do this also, you can give the item a unique id, then have an add to cart server behavior that is triggered on this item not being in the cart and just have it added. Please post back with any questions you have about any part of this.