There have been multiple ways of adding UWP (and WinRT before that) games and apps to Steam, but all of them usually require some time to set up. Fortunately, a new tool has been released that allows just that, but with a user-friendly interface.
UWPHook is exactly what it sound like. It creates a normal Win32-like shortcut for UWP-powered apps and games, to make sure programs and tools like Steam can work with them without problems.
For example, Forza Horizon 3 or Halo 5: Forge are both UWP games and can't be easily added to Steam's library. This tool should fix it in a couple of clicks.
The tool itself features Googles Material Design for some reason and is fairly easy to use. Just press the "+" button and select the apps you want to make shortcuts for.