I think it depends on what option you chose when you bought the tickets.

There used to be an option of seven days to either Sea World or Busch Gardens and when Aquatica opens next month that is available another option but think you have to pay more to get more than one option included for the 7 day pass.

You don't have to go wait till your scheduled trip to DC to get your pass for the other park/parks. The pass is valid for 7 days consecutive days and can be either before/after or both of your trip.

Say for example your DC ticket is valid for the last day of your holiday, you can go to the guest services a week before and they will give you the pass (don't forget to take your reservation proof with you). I think you should be able to do it at any stage because they will just make the pass activate on a certain date so if you are passing you can call in.