There totally is.
- Open the project with the mockup you would like to re-use
- Open a new, blank project alongside it.
- Drag the mockup from the main project’s navigator into the blank one
- Save the project with the single mockup
You now have a template file that you can use anytime you need that mockup in another project. Simply open that project and drag the mockup from one navigator to the other.
Alternatively, you can go to the mockup in your project, and select Project > Export > Mockup to JSON and copy the JSON information to a text file. Then, when you need that mockup, you can import that JSON by going to Project > Import > Mockup JSON and paste the contents of that text file.
Let me know if that makes sense, Michael.