[Drama]

You could go with a sockets proxy, or a hook one. The idea is same, you will be a middle hand between all the packets that travel from client to server and vice versa, which will give you the power to prevent packets from reaching their destination, modifying them, and even creating your own ones.

In a nutshell
Sockets Proxy
You write an application using sockets, and make your game-client connect to it. Then you'll make your application ( the proxy ) to connect the game-server with the data it received from client. Proxy'll receive the response from server, which you'll send to client. Now, you can watch the packets, figure out which are the right ones and programmatically send them to the server / client.

Hook Proxy <-- If you're using c++, I recommend this option.
The idea's same as sockets proxy, but you'll inject a DLL into your client, hooking send/recv methods in ws2_32.dll. Hooking and injection goes pretty far, you can even start a form inside target process from a dll.

I didn't go into details with neither of these two, but you'll find all them at google, I'm sure. For the API hooking, I suggest using a pre-built library for that, such as Deviare, etc.