Prepare translation for add-on

Is there a way to prepare translation to add-on that is automatically added to database after files upload to ftp (bypassing csv import)?

Not really, no :( I recommend a CSV.

This is not exactly what you asked but I'm doing something like that in my add-on

  {
    language::translate(__CLASS__ . ':not_selected', 'You need to select a destination locker');
    $this->set_translation('pl', __CLASS__ . ":not_selected", 'Musisz wybrać docelowy paczkomat');
  }

  public function uninstall()
  {
    database::query(
      "delete from " . DB_TABLE_TRANSLATIONS . "
    where code like '%" . __CLASS__ . "%';"
    );
  }

  public function set_translation($language_code, $key, $text)
  {
    $check_column_query = database::query("SHOW COLUMNS FROM " . DB_TABLE_TRANSLATIONS . " where field = 'text_" . $language_code . "'");
    $check_column = database::fetch($check_column_query);
    if ($check_column) {
      database::query(
        "update " . DB_TABLE_TRANSLATIONS . "
          set `text_" . $language_code . "` = '" . $text . "',
          date_updated = '" . date('Y-m-d H:i:s') . "'
          where code = '" . $key . "';"
      );
    }
  }```

language::translate(CLASS.':yada_yada', '...') is something that is recognized by the translation scanner. Admin -> Translations -> Scan.

Why not instruct your client to scan the platform for new translations?