Add products to order

Hello, Im requesting a new function to a e-commerce system called PrestaShop ([login to view URL]). There is one important thing this great system is missing, the ability too edit customer orders. It is possible to delete products from a order, but not possible to add new products to the order. So what im basicly requesting is the ability to Add new products to an existing order in the system. But there is one catch.... Since the system uses "modules" each country and store-owner uses their own payment methods. You need to create this future so its possible for anyone using this system to implement this function on all payment methods. If you have any questions or need more information, don't hesitate to contact me. Regards