Here I will show you how to unlock and use all DLC outfits without purchase from the Ultimate Survivor DLC which you can purchase on Steam. Due to limitations you can only do this for singleplayer unless you send the person you want to play with your Data0.pak file. So this technique works by unlocking hidden game files. Dying Light already has the DLC content installed, but it normally cannot be used because it is not unlocked by purchase of DLC.
Step 1: Goto Game Directory
This can be done by Right-Clicking on the game in your steam library and clicking on "Browse Local Files" in the "Local Files" tab.(As shown above)
Step 2: Create a Backup Folder
Just so when you are done using this mod or you mess up, you will have a backup of the orginal Data0.pak.
Step 3: Drag Data0.pak Over to Backup
Step 4: Drag Over Player Appearence Files
Open Data0.pak in either folders, goto Data, scripts, then drag over all the playerappearances scr files to the backup folder.
Step 5: Open Files
Open playerappearances.scr and playerappearances_dlc?.scr (Where ? is the dlc number, 9 is Ninja outfit, 10 is Urban Explorer, and 11 is Secret Agent) with your script editor of choice. I used Notepad++ but you can use regular notepad which comes with Windows.
Step 6: Copy Over Script Lines
Copy over the first lines(as shown above) from the playerappearances_dlc?.scr to the shown location above(Between the first and second lines). Then copy over sub unlock line found at the bottom of playerappearances_dlc?.scr then paste it somewhere in the sub unlock in playerappearances.scr
Then place the saved playerappearances.scr in the Data0.pak file in the Dying Light directory after you delete the old one.