Client-side added behavior

'm trying to design a system where a class would be defined in a project, be referenced in another and have new functionalities in the latter. Is there a pattern for this?

Context: I have a game that has items in a common project. Both the server and client reference this same project so I can have the item StaffItem in both the server and client, making it easier to serialize and deserialize between the two. The problem is, I can't redefine the StaffItem class in the client, since it will change the server's perspective of this class. I'm trying to find a nice way to add, for instance, the rendering to the client-side view of the class (added code for textures and all that).

I'm almost at the point of giving up and simply putting the rendering code in the common project, and stubbing it for the server. Any pointers (hehe) would be appreciated.