It is not illegal to ask you if you're willing to modify the agreement. You can always say no to it.
The only time you might not have a say is if you're a month-to-month when the landlord wants to change its terms. In that case, landlord will serve you with a 30-day Notice of Change of Terms. You must either agree to it or terminate the lease and vacate the premises.
I agree with Kenny Tan's response. The landlord can ask you to sign the addendum but you are under no obligation to sign the addendum, especially if it materially alters the terms of your lease which is already in place.