Just call and ask them if you can add a day or if this would mess up your package since it doesn't start until the 20th. They may let you since your original booking was during the special if there is room. The only way to know is to call. You probably will have to book two reservations. If you do this you may or may not have to change rooms. You won't know for sure until you chaeck in the 1st night. Why not stay 1 night at a DTD hotel if they won't add on and save some $$, then move to your hotel at WDW the next morning.