If the OP wants
Free Dining as well, then this won't work. Even if you add someone 72 hours in advance, they will be tied down to the same ticket obligations. The only way for what you're suggesting to work is if they don't want Free Dining, and they add themselves to the reservation after arrival, which is where ME issues may arise.
The only other way to be free from Free Dining's restrictions is to book a completely separate room, but that would likely be more expensive than paying for and pocketing the 2 day base ticket.
You can add days to the rest of your family's tickets during the trip.