The only sensible way to retrofit is to get a double side rear seat that has it and replace yours (though it needs to match fabric etc). Would have thought a breakers would have one. They arent that common as they were an option on A6's. I replaced my full interior (C5 rather than yr C6 model) with that from an RS6 - where the ski load was standard.
By the way, do u really want the ski load thru or just the open one?
Taking out the rear seats is easy (plenty of guides on internet), just be careful with airbags if they r fitted.
Good luck.