Adding days to someone else's tickets

[wdwlibrarian]

I'm traveling in a few weeks with a big family group. We all bought 6-day tickets. But DH and I have decided to stay two extra days after everyone else leaves, and so have my parents. DH and I will arrive first, and we plan to kill some time while waiting for everyone else to arrive by going to Guest Relations at Disney Springs and adding extra days to our tickets. Does anyone know if they will let us add days to my parents' tickets too while we are there? I won't have their magic bands - but their tickets and reservations are under my MDE (they don't have their own account). I figure I can do upgrade any tickets under my MDE, but wasn't entirely sure.