Yes, I had paid it off for about 3 weeks before adding the DDP.
But that doesn't make a difference. If you still have a balance for the MYW plan without the dining, your balance will simply be updated to include DDP.
Be aware that if you do add the DDP, and you've already received your travel docs w/ vouchers, etc. they will send another packet to you, assuming it's early enough for them to still mail it. It was a little confusing to us why we were getting the second packet, until I noticed that the vouchers for the reservation noted the DDP on them, while the first set we received just said MYW plan.