It is theoretically possible for someone to crack Steam DLC, bypass the security measures and protections put in place by the developer and allow the DLC to be used without purchasing it. However, breaking Steam DLC is illegal, and doing so would violate Steam's terms of service. In addition, Steam has sophisticated security measures to detect and prevent unauthorized access to its games and DLC, so attempting to crack Steam DLC would be difficult and risky.
Furthermore, even if someone could crack Steam DLC, using the cracked DLC would still be against the law and could result in legal consequences. 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.