In general, that's the only choice if you're adding on via Disney, though if you buy enough points they are willing to sell you points in another use year. Via resale, you can buy any use year you want. However, you probably want the same use year unless you have some special situation.
if it is a true "add-on", it MUST be the same UY month. if you buy a different UY month, that is a completely new membership, not an add-on to your current membership.