The "Create_item" property only works with the "Eat" and "OpenBundle" classes, and those classes can only be used with an item's actions ("Action0" or "Action1").
You could do it with a buff or perk, but you would need a triggered effect action that gives the player items (Dukes in this case). I looked in the decompiled code, and AFAICT, the vanilla game does not have such an action. You would have to create one using C# and DMT.
Also, AFAIK right now there is no way for the buff to determine which entity was killed by the player. So even if you did create that custom action, it might not do what you want.
If you want to give the players Dukes when they kill zombies, then you might be better off just dropping them in loot bags. That is still a ton of work, but it is possible to do by modifying the XML. You would create a custom loot container for each zombie type (or for each amount of Dukes to drop) and assign that to each zombie with a 100% drop chance.