My GTI came like yours without an armrest. They are easy to retrofit.
You can do just the rear arm rest part, but to complete the mod properly and make the rear vents work in the arm rest, you will also need the air vent pipe that fits under the main centre console section. It will connect to the a blanked off hose up behind the Ash tray area and the other end to a pipe in the arm rest if left not taken out already.