Pirating, or using unauthorized copies of games or DLC, is illegal and can lead to your Steam account being banned. In addition, pirated DLC may not work properly and can cause problems with your game, such as bugs, crashes, or other issues.
It is always best to support the developers of the games and DLC you enjoy by purchasing them through legitimate channels, such as the Steam store. This ensures that you have a legal, working copy of the game and DLC, and it helps support the continued development and improvement of the game. I would strongly discourage you from attempting to add pirated DLC to The Sims 4 or any other game on Steam.