1- Add the game's .exe on Steam as usual.
2- Check the game properties on Steam. On 'Target", and after a blank space, add the path to Origin's .exe. For instance, the Target on my Battlefront beta looks like this:
"D:\jocs\Origin Games\STAR WARS Battlefront Beta\starwarsbattlefront.exe" "D:\jocs\Origin\Origin.exe"
3- Go to Origin application settings. In "General", uncheck "Automatically launch Origin when I log on to Win”. In "Origin in Game", uncheck “Enable Origin In Game” to disable Origin overlay as it can give you some troubles with Steam overlay. In “Advanced”, check “Automatically exit Origin after closing game”. You will only need to do this once.
With this, when you launch the game from Steam, Origin will also be launched, then the game, and you should have full access to Steam overlay. Once you exit the game, Origin will be automatically closed.