In short, no. I don't think it can even be done like an actual three row seat 4Runners unless a lot of modifications are involved because it looks like the floor pan of the two row seat models are different by the second seat. Three row seat 4Runners, the second row folds forward differently. The seat backs drop and the entire seat flips forward towards the front seats, much like the GX470's. Three row seat 4Runners also have a recline feature on the second row seats, whereas our two row 4Runner second rows the seat backs are fixed in one angle.
IMO, I think converting a two row a three row isn't possible.
The only remote possibility is if the provisions for the third row are present behind carpet and panels on a two row 4Runner. You might be able to swap out the rear cargo area panels to the three seat model and maybe swap the seats in, but access to them will be difficult because of how the second row folds. My friend was able to put in Escalade third row seating in his Tahoe Z71 (which isn't available with the third row) by getting the mounting bars and cutting out the carpet.