I suppose it's possible but very difficult, because it would be necessary to have access to all the original files of the game, for example, when you download a game, is only the necessary files for you to connect to the server and play but with all files, especially those used to create the game and turn it on, you can edit it, create your own items and more, then the answer to your question is yes, it is possible to create a private server, but as I said, will be well difficult.