I think at OKW it doesn't matter at all. When I tried to do this at a regular room, when we had DDP...Disney did let me do it, but they told me if I added them ahead of getting there, then the extra person woudl be charged for DDP for the entire stay, AND we'd be charged the extra adult fee for the enire stay. In our case, we has multiple rooms, so one person was going to be staying part of the time in one room, and part of the time in another room. The computer wanted to assign the person to both rooms, which then created a conflict.
I don't think all of the above applies in your situation, becuase you are DVC i don't thinkt he extra adult fee applies. If someone is reading this who is not DVC, it might. If for some reason disney does want to charge you an extra adult fee, then below is the way to avoid it. It's also likely that Disney has fixed their booking system in the last two years.
What we did to get around the situation was to wait until we were AT WDW. Once you are there the resort CM was able to arange everything. I've also once or twice talked directly to ME folks over the years to make sure they had all our arrangements correct. That's another avenue if thins are not the way you like them.
The ME folks strongly prefer a minimum 24 hour notice. Last time i checked, You need 48 hour ntoice if you want to add a person to the DDP.