One problem I had with trying to add the DDP at a later date: I had originally booked my package on a code. By the time I decided to add DDP, the booking window for code I booked with had expired. The only way to add the DDP was to cancel my discounted package and rebook at regular package rates (there was no code available at that time). It would have cost me double what I originally paid! So I obviously chose not to add the DDP at that time. Yes, I ended up spending more money on food than if I would have included the DDP from the beginning, but it was still a ton less than if I had had to rebook once my code expired.
Long story short: IF you booked on a code, call Disney and ask about adding the DDP, it may end up costing you a lot more depending on your unique situation. It never hurts to ask!