It sounds like a simple job for anyone with a metalwork lathe or could possibly be done with just a drill press. I don't think you need a chuck if all your sanders have 6mm shafts - you just need a short length of round or hex bar with a 6mm axial hole and a grubscrew or two to grip them. The other end needs similar holes to fit the shaft on the motor. If you are lucky you might find some thick wall tubing to start with.
If the motor shaft is thick enough it might even be possible to drill a 6mm hole into the end of it, but I imagine your motor would be less chunky than that.