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.
Select "Contracts" > "Contract terms" and select the contract terms you want to update.
Select "Product selection" from the tab on the left of your screen and click on the plus symbols next to the products you want to add to your contract terms. Remember to save your changes.
All that's left for you to do is to update the existing contracts with the sellers that use those contract terms.
Select "Sellers" from the tab on the left on 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 select "Propose amendment". Now all you need to do is to wait for your sellers to accept your proposal.