I added the 3rd row to our old 2005 Explorer. The seat belts shouldn't be a problem as the upper mounts will be behind the trim panels, and the seat anchors will most likely act as the lower anchor point for the buckle (just look at a parts diagram to verify).
As said above, the rear trim panels will have to be changed out. I modified the existing trim to fit the seat belt hole and trim piece, but it never fit completely flush (not bad, but not perfect).
You will need new rear floor inserts and carpet floor panels, but the bolt holes will be in the floor for everything. The biggest pain will be getting all the correct fasteners for all the floor panels, seat belts, etc.