Yes, this is possible.
I have done this modification before by using one of the extra fields in the product table to hold a affiliate add to cart link url and then put a check into products.tpl and product.tpl to change the add to cart for a buy now button.
The buy now button has a href of the extra field param url.
I hope that isn't too much code speak for you.
Give me a shout if you want the mod done - would take about an hour.
__________________
ex x-cart guru 
|