Pick the closet nearest to the Plenium. Cut a hole in the floor in one end of the closet. Cut a hole in the ceiling directly above the hole in the floor. Run Metal Duct from the Plenium to the closet, up through the closet to the ceiling and into the attic. Once you are in the attic, you can use flex ducts or other types of ducts to the registers you will have to cut into the ceiling of the room you want to Heat or A/C. If you use flex pipe from the Plenium to the closet and up to the attic, it will collapse. Use rigid metal duct work only until you reach the attic.

Return air will be a problem, leave the door to the room open a crack and the air will sweep out of the room and into the rest of the house, using the air returns in the rest of the house.

I have a room that was built on a deck so there is no access underneath. I have forced air that runs through the crawl space and into each room from the floor. I would like to know if its possible to add a register to the room somehow.