To elaborate on Dan’s response, in other programs you will be familiar with the idea of importing house styles or libraries into your project (and this will become possible in future versions of Dorico, too), but for the time being in Dorico you do it the other way around: do File > Save As to save the project that contains your wanted master pages under another name, then delete all the players and flows from it, then either write your music directly into this new file, or if it already exists in another project, do File > Import > Flows to import those flows into the project, where they will then assume your existing master pages.