I agree with your logic! It makes sense to me that they could just add on the package.
I was able to switch out my regular 3:00 lunch reservation for 8 with a show package at the same time, but it wasn't easy. I did it online. At first, there wasn't any availability for a party of 8, but there was availability for a party of 4 at an earlier time. I canceled my regular reservation in order to book the show package for 4, just assuming I would split up my party. When I went to login under the other account to secure another reservation for 4, I checked first for availability for 8. Lo and behold, there was my lunch/show package for 8 at 3:00, the time I had just canceled. It's such a complicated process, and it really would make so much more sense just to be able to add it on to an existing reservation.