Yes,
DVC will only sell fewer than 150 points in the same Use Year as your master contract. If you have more than one contract with different Use Years, you can select which will accept the add-on.
If you buy a small contract thru resale, it's technically NOT an add-on since it is a stand alone contract. In some cases, we've had reports of DVC combining small resale contracts with existing contracts (by request) if the Use Year and home resort are the same as an existing contract.