No, you do not have to sign an addendum to an agreement or lease. No one, including your landlord, can force you to sign a contract (and that's what a lease, or lease addendum, is); furthermore, the landlord is (like you) bound to the terms of the original or existing lease. No party can force the other party to revise a contract/lease, including by an addendum; you would have to voluntarily choose to sign an addendum.