Hi @dwaltho and thanks for getting in touch about this!
The best way to demonstrate this is to create one mockup for each state you'd like to demonstrate. In your example, you would need to create one mockup with only the menu and then another one with the additional sub-menu.
Once all the needed mockups are created, you can easily link them together so that clicking on the desired item goes to the page with the sub-menu. You can learn more about our Linking feature here.
Hope this helps! Please let us know if you need anything else.