In the base Magento Marketplace module, the admin can add the product into the Seller Account by going to the Admin Panel and following the path Marketplace Management > Manage Seller > click on any seller name, and on the left side you will get menus and from those menus click on “Add Product”.
Here, the admin will enter the product id and if the admin wants to add more than one product then enter product ids by comma-separated.
Note- Here, the admin can assign one product for one seller or vendor only. Suppose there is a product A and there are two sellers S1 and S2 then the admin can assign Product A to either Seller S1 or to Seller S2.
If the admin wants to assign products to multiple sellers then you can check this module- Magento Marketplace Seller Price Comparison