I have always understood that DDP can only be purchased if you have a package (room/tickets) but I've seen threads lately where It says you can add DDP to your room only as long as you do it prior to arrival??
We are arriving in 10 days on a winter discount room only - can I add DDP or no?
Keep in mind, you're changing a RO to a ticketless package. Which means you'll need to pay the balance with throw and DDP in full and be refunded your RO deposit.