Packet editing is not a very simple concept at all. The way WPE works is to inject a DLL into the host application, which hooks onto the winsock functions: recv() recvfrom() send() sendto() etc. What it does is reroute these functions into some function within the DLL that modifies the packet. This is probably also your best shot at creating anything close to a packet editor.

Although like Cpp said, for JJ2 packet editing is very unnecessary since every function is provided very nicely to you already. Even if you really need to edit the packets for whatever reason, there are already nicely laid out functions in JJ2 assembly which you can hook directly, rather than the winsock functions.