To add on the dining package, you must have booked a room & ticket package first. So if you booked room only, you cannot purchase the dining package without also buying tickets.
That said, others in your position book a room with a 1 day base MYW ticket and add the dining package on to that. Keep in mind that you will be paying for the room at rack rate (not a discounted rate), so do the math. It may be less expensive for you to book an AP-discounted room and pay for food separately.
As a passholder, you are eligible to purchase the DDE (Disney Dining Experience) card for $50. That gives you 20% off food and beverages (including alcohol) at most table service restaurants and a few food courts. You'd only have to spend $250 at participating restaurants to break even on the purchase and the card is good for a year.
Best wishes-
