You can toggle an Add-in's sheets visible.

From the VBA Editor:
  • Select your Add-in in the Projects window (Ctrl-R)
  • Remove its password if it has one
  • Select the AddIn's ThisWorkbook object in the Project window
  • In the Properties (F4) window, change the IsAddin property to False
  • You can now see and edit the Add-In's worksheets
  • Change the IsAddin property back to True when done editing the sheets.