What do you mean by it doesn't have a 3rd row in it currently? Does it look like it was built without one, or was the 3rd row removed and a large hole exists? I'd proceed with caution on modifying this one if you buy it.
In 2008 one could certainly get a standard length Expedition XLT without a 3rd row. But it had cloth interior. When you added the optional leather to a standard length XLT, the manual folding vinyl 3rd row seat was included automatically. Power folding was then an option on top of the leather option. In other words, Ford didn't offer leather seats without a 3rd row.
Is the current drivers front seat power? If not, I suspect the truck was a fleet/SSV that either currently has vinyl (way too often mistaken as leather) or has aftermarket leather installed. The mint EB two-tone interior you are wanting to replace it with will certainly have 10-way power and climate controlled seats. When you start getting into things like side impact airbags, advanced seating position sensors and occupant detection, there is a lot of wiring, and CANBUS / BCM programming to consider.
Wiring harnesses, even the pin outs on those harness, vary by year and even differ between early and late production runs. Third row could be the least of your worries. Without intimate knowledge of the Expeditions wiring, its not a project I'd even consider.