Maybe, maybe not.
Even if you didn't need to go from room only to package, sometimes Change Reservation works, sometimes it doesn't.
Give it a go, click the Change feature and see if you can add the day you need. If you can't, call.
Though adding a day is not like it once was. The new way they do it is book an entirely new reservation, even though they don't change the reservation number. It's set up so that in order to add a day you have to be able to book all days all over again. Annoying PITA, is that it is. I'd still start out trying online.
As for tickets, do you have some now, and want to add or just need to buy them? If you just need to buy them you don't have to switch to a package. You can just buy the tickets. If you have a package now, and want to add to both the resort and tickets you already have, then again, try online. See if you can. Call if you can't do it online.