mbanks is right...its not as simple as just "adding on" the
free dining promotion. You basically have to cancel your existing reservation and rebook with the free dining promotion. If, for example, you are booked at WL in a standard room and the FD offer only has WL courtyard view for your dates, you have would have to switch.
I guess it couldn't hurt to book now, but don't expect that just b/c you have a room reserved that you'll get free dining on that room/package if its offered.