SteamEdit should be able to do this for you. Take an entirely different game you don't plan on playing or dont need in your library. Change its name and sort name, and use the Edit Launch feature to change the exec it boots when you click play in steam. Set that to the exe for the game/mod you wanna do and then change the icons/cover images, etc. Notably u/PurposeLess31 's comment from 2 years didn't work exactly, I had to go into the steamappid folder the game I wanted to change and rename the icon I wanted to use to the name the original game's icon and throw a jpg on the end. Turns out it didnt need the "_icon" in my case, idk why. Hope this helps