I would have to agree with it not being too hard. It will take a bit of time (depends on the amount of code, complexity, required latency compensation, networking background, design, etc) but it shouldn't be too bad. For the most part you will just be copying logic over to the server and stripping some out from the client (depends on what it is and how you want it to work) for the basic structure.
vbGORE - open-source VB6 multiplayer ORPG engine