5th Wheel Hitch Adapter.

You might want to look into the Andersen Ultimate. It mounts to the gooseneck ball in the bed of your truck, and has another ball at the height where a fifth wheel hitch would normally be. You attach a small adapter to the FW pin which in turn grabs the raised ball.

The advantage is that the trailer pin is in its normal position, so no added loads are placed on the frame, as happens when a gooseneck extension connects the pin to the bed-mounted ball.

There was a fairly recent thread on this topic in this forum.