Is it possible to add multiplayer on a project once the game is already deep in development or finished? If not so, do you know any way to adapt the game to a multiplayer/coop game?
yes it just a question of time, so will depend on your experience and how complex your project is.
I would recommend to test a small section first like only main characters replication, then moved to ui and finally events for other system of your game. This way you will be able to judge an average required time to finished them or not.
Actually i was thinking about an open world where you could open your session to other player, quite like minecraft bedrock edition
If you havent yet started your proejct, i suggest to watch some tuto yt on multi, then keep these concepts in mind when implementing any system in your game.
This way you will know each implementation will be compatible to multi after you have completed the game in a single player ver.
But still this will put limitation and add a huge time dev for the multi.