Hi Mr Who,
It is possible to import resource strings via Import Toolkit. On "Step 2 - Select object type to import" select Resource strings in *Select data type to import drop-down, and then Import as and select your culture. It is only possible to import 1 culture at a time, so if you have multiple cultures in one .xlsx file, you need to run import multiple times. The only other thing is, if you don't see your culture in the drop-down, make sure that this culture is marked as "Is UI culture" in Localization app > Cultures tab.
The process of using Import Toolkit is described here in the documentation.