Yes anyone staying on site can purchase a dining plan.
FREE DINING is a promotion where you pay rack rate for you room and buy at least two days of tickets for everyone on your reservation, but they give you the dining plan for free (depending on the level of stay you either get the QS plan, the Reg Dining Plan, or you can upgrade to a Deluxe dining plan).
They may not apply Free Dining if you have a pre reserved stay with a purchased plan. The free dining offer may not include your hotel or even the room level at your hotel. It may not include the dates that you are going. So you cannot always transfer the FREE DINING to your reservation.
Sometimes you have to book a new reservation then cancel the old. (expecially if you have a room only reservation and want to change to a package)
FD is usually offered in the late summer and fall.