The documentation for CSV File Import says: "You can import issues with multiple labels by entering each label in a separate column." However, if you attempt this, in the "Select a field" drop down in the "Setup field mapping" screen, the Labels field is not available as a target field (Only "Attachment" and "Comment Body" are the available Issue Fields). It seems as thought the Labels field is not recognized by the CSV File Importer ass being able to accept plural values.