Yes, in order to create a listing to Rakuten, you will need an already existing Rakuten SKU or UPC.
1. Navigate to Inventory -> inventory management
2. Click the binoculars icon to click into a SKU number
3. Choose the listing attributes tab, choose Rakuten and enter the Rakuten SKU
4. Click on the Details tab, scroll down and choose attributes to enter the 'UPC' value
To find the Rakuten SKU, click here.