Yes, seems that it's possible to add the third row to a Trail after purchase. A few folks on T4R.org have done it. However, I recall that some of the weldnuts are not in the body, and need to be added. This is for things like safety belts and latch anchors for car seats. That's one thing I get uncomfortable about, personally. Also, the entire cargo area plastic needs to be swapped to do it the right way. Additionally, the 2nd row seats don't fold in a way on the 2-row versions that allows easy ingress/egress to the 3rd row.
One positive thing; there was some concern on T4R.org that the side curtain airbags on 2-row models don't cover the 3rd row. From a few crash/rollover pics, I don't believe that to be the case. The side curtain airbags I have seen look to extend all of the way to the back window including on the Trail model.
Dan