Yes, it is possible to import via a CSV file.
To ensure your file follows the format required, visit https://my.flightradar24.com/settings/export and download a CSV file to see how it should be formatted.
The main rules:
Date, Origin and Destination are the only mandatory fields.
The date format must be Year, Month, Date format (2010-04-16).
The system should understand the ICAO/IATA codes and upload them, you can use "EGLL", "LHR", LHR/EGLL" or "EGLL/LHR".
After that you can upload your file by clicking on “Settings” on the top right of the screen, then “Import” or use this quick link: https://my.flightradar24.com/settings/import.
Finally, press “Import CSV” and select your file to start the import.