Can third row seating be added to a 2000 Mercury Mountaineer?

Oh, dude, adding third-row seating to a 2000 Mercury Mountaineer is like trying to turn a bicycle into a motorcycle - technically possible, but a whole lot of work. You'd have to find compatible seats, modify the interior, and probably spend a good chunk of change. So, yeah, you can do it, but it's not exactly a walk in the park.