Both DVC and AP holders can add a dining plan.
Basically if you book through Disney you can add a dining plan. If you have a room-only bookimg you can call and ask for a ticketless package, which allows you to add a dining plan.
If you book through a TA you can add a dining plan. But booking through third party sites like Priceline, or using a special convention or group rate disqualifies you from adding a dining plan.
Currently it seems those with a special AP rate cannot add the new one. Nor can anyone booking through the U.K. site, although it is showing up there so I think it might be available eventually.