I would add it as soon as possible. Were you planning on making any reservations? If so, you should book your DDP as soon as possible so you can start calling for reservations. Many of the restaurants fill up quickly, and you'll be waiting for hours if you try to just show up, especially during peak meal times. It's really the way to go!
Whether your reservation would allow it depends on how your travel agency (USAir) booked the package.