Is it possible, to export Master Page Layout to there projects or save them as default?
Unfortunately not at the moment but according to the Dorico team this feature will come in the future.
What you can do now ist to create an empty project with master pages to your liking and save this project.
You can then later open this project and create or copy new flows in it or import flows from other projects.
Thanks.
It would be a real important feature.