If you have your own game and/or matchmaking server then there is nothing stopping you from having people on Steam play against people on other platforms. Just pass through their Steam ID with the login to your server if you need to track if they are from Steam or not.

Thanks guys.

My resources are limited and I am not a hot programmer so making our own solution or bridges might prove tricky.

Therefore I have also been looking at other third part backend systems/services but I am finding it hard to understand if they will actually help me with getting around NAT issues, which is one of the main reasons I am looking at steam.

What I was planning was to have just a very limited dedicated server that would just host the lobby itself, and it would enable players that are hooking up to challenge opponents from the lobby list and then the server would connect them together and get out of the way, letting Unreal do its Listenserver-client thing with the two players themselves.

I have been looking at for example Photon, Playfab and Gamesparks without being able to actually figure out if I could do this using them.