If a friend owns the DLC, Steam Family Sharing allows you to play their library. Launch Dying Light 2 from their account, download the DLC, then switch back to your account. The items will persist in your stash.
While searching for a is common for players on a budget, the risk of losing your entire Steam library or infecting your PC is simply too high. With the amount of free content Techland drops via "Community Updates," there is plenty to do in Villedor without taking the "shady" route.
Before considering risky third-party tools, ensure you have utilized official methods to obtain content for free or at a discount: acidicoala/SmokeAPI: Legit DLC Unlocker for Steamworks
If you previously attempted to use an unlocker and are now experiencing crashes, here is how to fix your game: