By definition, an "Add-On" is specifically an addition to your existing contract. It keeps the same contract number and has a suffix added. Since your existing contract has a specific use year, any addition retains the same use year month. (It can however be at a different resort)
If you wanted a different use year month, you need to purchase a totally new contract (which would have it's own membership number). You could purchase the new contract from Disney (160 point minimum), or from resale (no minimum, just whatever is available).