I think there is going to have to be a little bit of sliding (not giving away). I'm currently trying to add 1 seat, and my guess is that if there isn't a seat directly next to mine, but there is one down the row at the end or somewhere in-between, then they may (I'm only guessing) shift the ticket holders between me and the seat in order to make it work. It wouldn't bother me if I shifted a seat or two max to make the row fill out correctly.
I'll just be really pissed if there isn't any way to get my extra seat blocked with my originals considering I was told the end of last season when I bought it (somewhere across the stadium) not to worry about the process, and I've been paying installments already.