At the moment you will not be able to list products to Rakuten from SellerExpress. However we will be offering an additional service which we can list your products to Rakuten for you. If you would be interested in that please do get in touch with support via email at support@sellerexpress to request that.
If you would prefer to list the products yourself you can go ahead and do that directly on Rakuten. To link the products to SellerExpress you can then add the products manually on screen (See here for more details on adding products to SellerExpress), and you will need to add your Rakuten Sku into the Rakuten sku field to sync them up.
Please note: Initially you will not be able to add your Rakuten sku in bulk using the imports. When initially adding your products please do get in touch with the support team via email at email@example.com for help adding your products in bulk.