Wurst use external files and then insert the code on demand.
So you can take existing code, and then add wurst on top.
However, if you open the map in the editor, add some additional GUI, the world editor save will overwrite the wurst code.
So if you want to go with wurst, you need to use it all the way basically.
Until WEX has it implemented, then it will be absolutely awesome.
In theory you could just add the wurst code every time after you save after editing in the WE, that would work. But would be very annoying.