The trick is very simple: you must save the data of an account that was registered to WBID

Thanks to the user Blackwolfpt for providing his files.
https://www.mediafire.com/folder/p8trqpuxb4vj0/Batman+Arkham+Origins+100%

The files you need to change are in
C:\Program Files (x86)\Steam\userdata\[Steam ID]\209000\remote

Since the WBID account is "registered", this skin will remain even if we delete the save file. (You can check it out in this challenge mode match)
https://www.mediafire.com/folder/djxg9uynkhtkj/batman+Arkham+Origins+0%
This folder is for new users or those who haven't made much progress, so they can complete the game themselves.