I have started to explore unity, their free add-ons, and built-in libraries. That made me came across some add-ons that can make your single player game made in unity become instantly multiplayer to play with friends, generating rooms so everyone can connect.
That made me think if any game can run in a multiplayer mode with a dedicated server even outside unity.
Does anyone know, if it is possible for single games like super Mario bros., insidious, etc just with the game files installed have some 3rd party software that allows you to create that multiplayer that can connect via private server and play with more people at the same time?
If you are wanting to be able to run a number of instances of the same single players game on a server then yes.
If you are wanting to have players interact with each other without modifying code then no.
Ok, so I had a quick look at the Sims 4 multiplayer mod. The mod is written in Python and works by sharing access to one players sim database. So it is not a multiplayer game in the true sense. You are basically creating a number of sims in the one game and then share control of the one database for the sims with up to 4 other players on a network connection using LogMeIn Hamachi which is a virtual private network application.
Another way to look at it is you are mirroring the one game but have multiply game controllers.
Everything is possible with enough dedication, money and time...
More realisticly, no, there is no general formula for turning a single player game into a multiplayer game. The solution need to be adapted to the specific game (or the technology that the game uses). If you have access to the original source and tools that was used to make the game it will be much easier, which is why you are more likely to succeed with a free open source game than some propretary game where you only have access to some compiled and undocumented version of the game.
My words based on my old experience so my answer is no why? because multiplayer mode made by the game developer if the game doesn't include it so you can't add a multiplayer mode, or you can create a program to run the multiplayer mod i will give you a simple example GTA:sa made for single player mode but SA-MP released for it, to let players join together, so the answer no and yes if you know how to code and build programs using C++, Java and other languages