OK, so I've looked into adding a third row to my HZJ80. Quickly decided against it. It's possible to do, but to get the same result as factory, you'd have to cut holes into the body for the "wells" that hold the bars the seats lock into in the floor. You'd also need to cut those wells out of a rig with them and weld them into your vehicle. You'll need to cut the brackets off the wheel arches for the third row seats too, and weld them in as well. Then as @LINUS mentioned there'll be no captured nuts in the pillars for the seatbelts, so you'd need to drill them and fit mounting points for the seatbelts. Doable, but a PITA.