if you had to purchase a new master contract, it wouldn't be an "add-on."
you can add-on at any resort you like...VWL, BCV, whatever...but it has to be for the same use year month. also, you can't combine pts to book at the 11 month window. your new (VWL or whatever) pts won't be good for booking AKV until 7 months out and the same is true for AKV pts at the new add-on resort.
you can add-on 25 or more points if you pay cash, and 50 or more pts if you finance. (and there is no incremental requirement: you can buy 27 pts or 31 pts or 53 pts, if it suits you.)