If your PIN discount applies to a room/tickets package, then yes, you can add the dining plan to the package.
If it applies only to a room-only reservation, you cannot add the dining plan. The plan can be added only to a room/tickets package except for special circumstances, like the military discount.
If you must buy the room and tickets separately, even if you buy both through Disney, you do not have a package reservation and cannot add the dining plan.