How to add Minecraft Bukkit to existing server?

By setting it up do you mean port forwarding/hamachi and all that junk? If so then all you have to do is take your world file and op list and any other files you have edited like the setting/options file and put them into a new folder. Then, download bukkit and place it inside the same folder. When you run bukkit it should use your world file and options and everything. It will have the same ip and everything so you will not need to port forward again or anything like that. Hope this helps.