No, but you can at least setup accounts for your external vendors and track their sales and pay them commissions as needed. You can assign zero, one or many vendors to a single account and it tracks each vendor separately. You can also allow those accounts to control their inventory by showing/hiding it, setting prices or changing inventory quantity. About the only thing NOT supported is those external accounts creating products. An embedded App is available for you to test out here https://tkitty.herokuapp.com/shopify