Using the CSV import tool in Store Commander, you can create new products and associate them to various shops as well as deciding on the default store for these new products.
There are 2 specific fields to use when importing in multistore mode:
id_shop_default : ID of the default store for those products
id_shop_list : ID of all stores onto which products will be added
The corresponding mapping shall be:
You'll find the shop ID in the Multistor Sharing Manager in the Property column in Store Commander, or in the Multistore section of your PrestaShop backoffice.