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Perhaps if you were to explain what it is that you are trying to achieve this request might make more sense.
The purpose of a sitemap is to aid with search engine indexing (although it is not considered a requirement for dynamic sites such as phpBB) but you want to have your private forums included which, to me, sounds contradictory.
David Remember: You only know what you know and - you don't know what you don't know!
I now no longer support any of my extensions but they will start to become available here
The built in permissions system allows you to have visible and hidden forums depending on whether you're registered or not and which groups you're in. It sounds like you want to replace all of that with just a list of URLs which would be super hard to navigate.
-:|:- Support Request Template -:|:- "Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb"