Seeing how complicated some MMO servers can be, I doubt it would be easy to set up a private servers of such a system. Most of private servers are probably not packaged and properly documented for the common geek’s understanding as you can image.
A way to build a private game server is reverse-engineering. I know at least 3 examples of private servers being created this way for example Ultima Online, Final Fantasy XI and Maple Story servers.
It all started with people having a deep knowledge of the game server mechanics. Damage dealing equations, experience obtained, the impact of each stat on each action, monsters behaviours … it can all be figured by the game server community.
Another important aspect private game servers is the communication protocol. Average private game server has to understand what the game client sends, and has to send him expected and meaningful answers. It might be secured in various ways, too to make everything work in harmony.
If you can have a clear communication with the game client and know how the game server behaves, you can build a working prototype for private game server. If the official game team managed to have a working server, there’s no reason you couldn’t do it. Keep in mind t’s not an easy task though as you can imagine, but it can be done and it has been done again and again, hence all the private game servers.