I apologize for any inconvenience caused, but for now this is not possible.
However, you can always do it in bulk using our Premium add-on ATUM Export Pro. With this add-on you will be able to export any kind of data in your site, in this case Multi-Inventory products, import it on a spreadsheet to edit it in bulk (in this case assign countries) and then import it again to your site.
I leave you here the documentation of the add-on where you will find useful info about how it works: https://forum.stockmanagementlabs.com/t/atum-export-pro-documentation
And the link to the demo site in case you want to try the free trial: https://www.stockmanagementlabs.com/addons/atum-export-pro/
Hope this helps,