Once you add the DDP to your reservation, it's no longer "room only" since the DDP is indeed a MYW package; therefore you are going to need to purchase tickets (1-day base tickets at the minimum) for each member of your party as well at the time of "upgrade".
We've "sorta" done this before, especially since Disney expects you to pay off EVERYTHING 45 days before hand. We normally book a package and just have 1-day base tickets. After we check-in, we upgrade out tickets to 7-day park hoppers/water park and more tickets. For just two people, the difference in tickets was $520ish; and it was nice having those extra 45 days to save up for it!
We usually end up capitalizing on packages rather than room only due to better savings; so haven't had to upgrade a room only reservation.