Thanks for clarifying. I can't speak for the RealAir version, but I'm fairly certain that there has never been a real world normally aspirated Duke. It was built as a pressurized cabin high altitude aircraft right from the beginning.
That being said... in my constant strive to provide custom solutions for my users, you could make this modification on your end without substantial difficulty. It's not as easy as some modifications I provide for users, but possible. Let me know if you want me to walk you through it. In the meantime, you could also just trigger a dual turbocharger failure in the failure menu of the tablet interface, and you will have yourself a normally aspirated aircraft!