You can add new products to an existing marketplace contract which enables your sellers to sell those products for you. You do that by updating the contract terms that are connected to the marketplace contract and adding the new products to the terms.
Click Marketplace > Contract Terms and select the contract terms you want to update.
Click the Product Selection tab at the top of your screen and check the box next to the products you want to add to your contract terms. Remember to click Save and the bottom of the page.
All that's left for you to do is to update the existing contracts with the sellers that use those contract terms.
Click the Sellers tab at the top of your screen Tick the boxes next to the sellers you want to update your contract with, write a comment with a brief description of the update and then click Propose amendment. Now all you need to do is to wait for your sellers to accept your proposal.