You can make an existing app a Base App.
If you do this then that app will become one of the choices for building from when you create a new app.
To do this go to the Edit App/General tab and check the checkbox that says "Public Base App".
Go to the bottom of this page and click "Update". You will get an error on top that says
1 error prohibited this app from being saved
There were problems with the following fields:
- Store description can't be blank
This will then expose a tab that says "Base App". You can put placeholder text or the text you would like the user to see when they create a new app from your Base app.
Once you have filled this out go to the bottom of this page and click "Update" to save your changes. The text you enter will display when the app is picked in the store as shown below:
Your new base app will now be one of the choices when a user creates a new app in any account Workspace.