Can I Use Any Game That Is Not In The Games List?

If you want to play game like The Witcher 3 or Factorio that is not in Portableapps.comFormat already... You will have to create it yourself. But be careful about game with DRM like game on Steam, Origin etc. Because you can't launch them without the app that you bought it on. That why I talked about The Witcher 3 who is DRM-Free on GoG (meaning that you can Copy/Paste the game folder and play it without the GoG launcher normally) and Factorio give a zip archive of the game already on there website (when you own it). So conclusion you can't unless there is no DRM and created in Portableapps.comFormat.

PS: Check in the beta test sub-forum for game already created in Portableapps.comFormat like Minecraft or Need for Speed etc. (You have to own this game thought)