There has always been a feature in Steam to add non-Steam games.  
https://streamable.com/pukwz
For example retail or downloaded games like:
This could totally put PlayOnLinux and Lutris out-of-business
				
			https://streamable.com/pukwz
For example retail or downloaded games like:
- Unreal Tournament 3
 - Doom 3
 - Quake Wars
 - Indie Games
 - MMORPGs
 
This could totally put PlayOnLinux and Lutris out-of-business