An Addendum is used to add, remove or change a term(s) in a Contract. Normally you would address Seller paid closing costs up front in the Contract but if it didn't fit it or you were making athe change later it would be perfectly fine to use an Addendum form, a cocktail napkin or anything else for that matter. The important thing is that you and the Seller agree in writing.
If this new term is not in the offer you have submitted, then you probably need either an addendum, or a counter offer, depending on where you are in the negotiations.
Your agent will be advising you on the right form, plus you will need the Buyer's lender approval of the new terms, too.