Strings in .resx are independent on those in UI Cultures (these strings are in database in CMS_ResourceString table). I dont think there is a possibility to automatically import them to the database, however you can create (copy) the required strings manually and they will override the default strings from the .resx files.

I would suggest not to modify the .resx files as it complicates Kentico upgrades.

Hello.

Petr is basically right. You would need to create their copy using API. You would need to parse physical resource file and create respective objects using the CMS.SiteProvider.UICultureInfoProvider class.

Best Regards,
Radek Macalik