You can add missing seasons yourself. The procedure is explained in the FAQ:

The "Ended" and "Cancelled" TV Show Status now automatically disabled the "Add New Season" button. New seasons can't be created when a TV entry is set to Ended or Cancelled. [...] For ended/cancelled series with a missing season, you are allowed to temporarily change the TV Show Status back to "Returning Series". Just don't forget to set it back to Ended/Cancelled when you are done adding the missing content.