In radiant, place four script_structs, give TWO of the four either:
OR
Next, open up _zombiemode.gsc(If it isn't in your IWD then i'd add it at this point). In it find the function:
and replace its contents(code) with:
So it looks like this:
Done! Launch it in coop with a few friends and give it a try. If you have any issues feel free to post below.