I was wondering if it is possible to create a dedicated game server for a game that already exists and you don't have access to. For example, a game on steam that only allows multiplayer through hosting a game with your running client. A client can host a server but only while the hoster is also playing the game.

The question being, is it possible to write a server from scratch that can rep
lace the game client host with a dedicated server that does not require a player hosting it.

And if it is possible, is there any resources available on the process of developing a dedicated server for an existing game?