You can't get the free dining if you are already getting another discount. It's one or the other. If you are paying rack rate for the room, you can get free dining I think by just buying a one day ticket for each person in the room or that would be putting on the plan. In the case of the Poly I think the room discount is a better deal than free dining by far.
If you didn't mean free dining, but just the dining plan that you pay for, yes you can add it to a room only reservation, it doesn't have to be a package.