You can only purchase the DDP as part of a package, which means room, dining plan for each person for each day, and a one-day (at least) base ticket per person,.
If you have a room only reservation, you cannot buy the dining plan without changing the reservation to a package, which means giving up any room only code you might be using. (I have read about Disney Vacation Club members staying at DVC properties using points and buying DDP, as I believe they can by the DDP without a package. But don't quote me on that ... I'm sure there are rules. That may be what you read, too.)
There have been codes that apply to packages, so that is a possibility. I'm not totally confident about the rules for changes/upgrades after the 45 day mark if you already have something booked. Check with a TA on that one.