Is there any significance to which resort you buy the add on through as compared to your home resort? I assume you would just want to strategize on what the best usage year would be for the add on to coordinate with your main UY?
Unless you buy an entirely new contract (of at least 150 pts through DVC or any amount resale) DVC will only sell you an add-on that matches your existing Use Year.
You can add-on at any resort, but you only get that Home resort booking advantage for the number of points that you own at the given resort. Thus, if you have 300 OKW points and 25 at BCV, you can only use the 25 to book BCV at 11 months.
Some people with small add-ons will use banking and borrowing on their small contract to give themselves up to 3 years' worth of points to book at 11 months for a single trip. If you're uncomfortable with the point management this would entail, it probably makes sense to limit your add-ons to your primary resort.