add to shopping cart from one site to another

I am in need of someone that can do the following: I am unsure the method that needs to be used so i am doing my best to describe. I have a branded company website that clients can order products from. This branded website handles all payment and and orders, shipping and all. Since this branded company site does not allow for any customizations what so ever it have built a new website complete with all of their product listings. I am in need to connecting the product listings to automatically add to the shopping cart on the branded company website. I believe you can do this with passing session data, but again i am unsure. I am needing this functionally so that clients can view the product on my website and when they click add to cart, it automatically adds it to the cart on company branded site and from there they can choose to checkout or browse more products on the company branded site. Again the request for add to cart would be coming from new website and posted to company branded site (different url)