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I suspect this is related to your question here:
https://www.virtualmin.com/node/16567#comment-73417
I don't believe there's a way to edit those templates, and that's because Virtualmin rewrites them based on the current featureset of the Virtual Server.
However, you should be able to do what you're after by making changes to the global procmailrc file, inserting your code before the delivery is taking place.
-Eric