Once you make your initial purchase (via
DVC or resale) you can always add-on at a later date. Add on pricing will be the current DVC prices, and you may have to go onto a waiting list if you want to add-on at one of the sold out resorts.
For example, if you bought 150 at SSR, and then wanted to get 50 points at BCV. BCV is currently sold out, but DVC does reacquire points from time-to-time by exercising Right of First Refusal on a resale contract.
But there's no need for the waiting list if DVC is actively selling the resort, as in the case of SSR.
You can add-on as few as 25 points at a time. But if you want to finance the add-on through DVC, you need to add at least 50. And you can add-on as many times as you wish. You could buy another 25 in 2005, 50 more in 2006, and so on.
One nice things about add-ons is the contracts for each add-on are separate. So if you add-on 50 points in 2006, and 10 years later decide that you have more points than you need (kids growing up, not traveling to WDW as much anymore), you can sell a single add-on contract while keeping the contracts you are still using.