No, that can't be done.
The time you have to add a dining plan depends on the type of reservation you have.
On a package reservation (and you must have a room/tickets package unless you are an annual passholder or eligible for military discounts - you cannot have purchased the room and tickets separately) you have up to 72 hours before arrival. If you add the plan within 45 days of arrival you must pay for it at the time you add it.
On a DVC points reservation, you have up to 48 hours before arrival.
If you booked a DVC reservation prior to 10/14/10, you must pay for the dining plan when you check in.
If you booked a DVC reservation on 10/15/10 or after, you must pay for the plan at the time you add it to the reservation.