You can't add the DDP without a package (or so I've been told). You can call in, book a ticket less package with the ddp and then you can use the room only promo for the room portion, pay full price for your ddp and order tickets from somewhere with a discount such as
undercover tourist.
Did you book the play stay and dine package though? If you did, you might want to make sure that savings on the hotel will still save you more money than whatever package discount you got.
For my vacation I booked my room through Disney with the fun filled adventure promo, my tickets through undercover tourist and skipped the ddp because when I did the cost comparison it was cheaper to just pay out of pocket for my meals I'd planned.