You can add Windows Store apps to Steam as a Non-Steam game, although it currently won't work with the Steam overlay.
- Download UWPHook from here:
https://github.com/BrianLima/UWPHook/releases
(Look for the latest "Release.zip" Download) - Extract the zip file and moves the files to a folder on your computer.
- Run the UWPHook application.
- Click this button to load alist of all your installed Windows App:
- Check the apps that you want to add to Steam and click the "Export selected Apps to Steam" button.
- A prompt will appear. Restart Steam and the games should now appear in your library.