Anything is possible with enough $$$ and tolerance for risk. It's been done in a limited way in a 997 GT3:
http://rennlist.com/forums/997-gt2-gt3-forum/376327-rear-seats-installed-pics-2.htmlI am assuming you do not have the fixed carbon buckets in your car, as they do not allow sufficient access to the rear compartment to allow this anyway, without removing a seat. The main question about whether the seatbelt anchors in the rear are present in the 991 GT3 in the same way as other models has yet to be answered. I would assume they are, but don't know anyone who has taken up the carpet and looked yet. You might talk to some of the guys/shops doing cage installs who have removed the interior in the rear. The GMG site mentions their roll bar mounting "to the seat belt anchors at the B pillar" so I assume the GT3 "body in white" has the same welded anchor points as the other 991 cars, but just plugged or blanked off, though I haven't seen this confirmed anywhere, yet:
http://rennlist.com/forums/991-gt3-and-rs/754582-back-seat-in-991-gt3.html. There is a chance that the factory deleted the rear seat backing plates and anchors for weight savings.
The ultimate question would be whether you are comfortable with the risk of putting a child somewhere in a car that wasn't designed for them to be seated? It is actually illegal under a strict interpretation of the CA Vehicle Code, so I doubt many shops would be willing to assume the liability for performing such modifications. If you are comfortable with a "self-engineered" solution, then it is probably possible.
YMMV,
TT