In order to purchase the dining plan, you must purchase a package from Disney (or AAA). This includes room and tickets. You only need to purchase a 1 day ticket but everyone in the room must have one and everyone must be on the dining plan.
If you're planning on buying FL resident or seasonal passes, not sure if you can buy the one day and then use it to upgrade to what you want, but I'm sure someone will tell you.
Also, if you have any discount on the room (unless it's part of an offered package, like the one that just came out), you will be paying rack rate (or the discounted AAA rate).
Hope this helps.