Bob,
You would need to find a contract for sale with your same use year, makes it easier to keep track of one use year. The points are kept on a separate contract. i.e. your original contract # will show a .1, .2, .3 etc for your added on contracts. If you stay at OKW the point tracking will be much easier. We have contracts at all DVC properties and when making a reservation we need to make sure we tell MS from where to use the points. Actually the member site is great for tracking your points and reservations.
Adding on via resale was painless. The pain is waiting to see if your off passes FROR. Sometime DVC is quick and sometimes the are slow. We have only done two of our addons via resale. DVC has had some pretty good deals. Some contacts the seller pays closing, usually when you make an offer on a contract a escrow check will be required. There are several resale folks out there, another one is DVC By Resale, and the Time Share Store.
Hope this helps.