If you're not yet a
DVC member, then you have to purchase a minimum of 150 points if you're buying directly from Disney. So, they would have to at least 150 BCV points in the same use year available for you to purchase. You could not, as a nonmember, buy in at 60 points if that is all they had and wait for the rest of the points to become available, and they would not combine points from different use years to pull together the minimum 150 points needed.
Once you are a member, then you can purchase an add-on of as few as 25 points, again in the same use year as your (or one of your) existing contract(s).
The only way DVC will sell you a different use year is if you purchase a whole new contract of at least 150 points.
Edited to add: If you purchase a resale contract, the 150-point minimum does not apply. The smallest resale contract you will see will be 25 points and if you were lucky enough to get it without Disney exercising ROFR, you could become a DVC member with as few as 25 points. If you want to purchase points at BCV and you don't want to wait forever for DVC to have 150 points left over after fulfilling all the current members waiting to add on, consider buying a resale contract. To get an idea of what is available via resale, scroll up to the top of this page and click on "DVS Resales". You will see the list of DVC contracts available through The Timeshare Store, this board's sponsor.
Good luck!