I know that when we were looking at 01 tahoes (in 01 when they were new) we looked at some that didn't have the 3rd row seats, and you could feel the attachment points under the carpet. So, on those years it would be easy to swap in some 3rd row seats (I would think), but I would think only the 3rd row seats of that vintage would work. They're actually two split seats vs. a small bench.
I'm not familiar with what the 99's had though. But again, I would think the seat(s) would have to come from s sub of that vintage range. I can't even remember what exact body style the 99's had. The same as an 01? I know my buddy's 99 K2500 was the same body as our 01 Tahoe.