Normally, you can purchase an extra seat, if, for example, you need extra space. The extra ticket is specially designated as an extra seat. The question is whether AA will let you use a ticket issued in another passengers name as an extra seat. I suspect no, because this could enable non-travelers to earn AAdvantage miles, which is against the rules. You could ask AA if they'll change the name on the ticket to be an extra seat, but I also suspect they may not do this, at least not for free.