Not within Zencart, I don't think. I'm not sure I understand exactly what you're trying to do, but I still would wonder what would happen if I, as a customer, wanted to buy something from a 'local' category, and something from a 'remote' category - two separate checkouts ?

One possible kludge would be to create an EZpage for each remote category, using an external link to the remote site, and place the EZ page sidebox right under the Categories sidebox, remove the header and play with margins and padding so the two looked like the same sidebox.