You can only swap cables if you have the newer type of TG 16 pads. The older cables have a different wire configuration. So basically if you open up a pce pad and the wire colors are the same as the TG pad then you can swap them no problem.
However this does not work for a 6 button game pad. There may be a way to swap around the wires but I haven't been able to find any documentation on how to do so for a 6 button pad.
One easy option is to swap the cables on a DUO 5 player tap with a TG16 5 player tap. This will let you use up to 5 pce controllers on your US Turbo grafx including a 6 button game pad.