Heck, just get a 5" diameter piece of black gas pipe 24" long. Cost you $40 give or take, then weld that on to the frame of the Reese hitch. I dont see why the GRCS wouldn't ratchet onto that. You might weld some horizontal bead onto the pipe for texture.
You have to get it cut at a supply house, no threads. A ready threaded nipple of 5" pipe will cost you a fortune.