180 days from the day you check in- yes. You can make ADRs that are anytime during your entire stay. (so technically 185 days for example)
If you are a DVC owner you can always confirm the answers you get here with DVC MemberServices.
If you are renting, you will need to let the owner know you wish to have this added to your reservation. I did this for the 2 families I rented to- and asked them up front so they would see the notations on their confirmation.
I do believe the usual way to do it is to pay for PH and DDP at check in, these were the instructions DVC gave me when I called, to anticipate paying at check in.
PH and water parks are typically purchased as 1 ticket.
Be aware that you have to tell them in advance what you are purchasing.