Unless they've changed it this year, I believe you have to add
DDP prior to arrival date. Something like 7-10 days comes to mind. I remember this subject being discussed, at length, last year prior to the free DDP announcement (expected in April).
As to adding it to your reservation, you can. Last year I booked my room a few days before announcement was expected and then when they made the announcement I called and rebooked it.
I'm sure someone on these boards will know the exact time frame.
I would call them back and make your changes.