You can add on at any resort. The minimum add-on is 25 points if you are paying cash or 50 points if you want Disney financing.
If you own at two different resorts, when booking 11 months out you can only use points from the resort you are trying to book. For example if you purchase 160 points at SSR and add on 25 points at BCV and want to book BCV 11 months out, you can only use the 25 BCV points to do so. You can use banking and borrowing to accumulate up to 3 years worth of points, so in this case you would have 25 BCV points to use every year or 50 BCV points to use every other year or 75 points to use every third year.