I'm not sure if I follow the meaning of your question.
Are you trying to figure out how to put a 3rd seat back because you've got a kid on the way and you want them to sit back there in a carseat?
If that's the case, I'd say don't do it. I don't think I'd ever want to trust the safety of my newborn child to some jury-rigged 3rd seat in the back of a car that wasn't designed for more than 2 seats.
Y'know, you don't HAVE to buy a minivan to haul a kid around in.