Add to cart without switching sites (Global catalog)

I apologize if this has been asked before, I couldn't find the answer anywhere by searching the forums. I am trying to add products from the global catalog without switching to the individual store site in order to add the item to the cart. After I add the products from the individual sites and go back to the main site, the products are in the shopping cart (good) but I don't want to have to go to the individual sites in order to do it. I put together a rough mock-up with nothing fancy to illustrate what I mean.

http://dev.wallemart.com/

Also is there a way to put the providers name associated with the product and filter products by location of the providers? If this requires some extra customization(s) we would be interested in getting a quote.

Thanks!
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Darcy