The Overlay page of the Printing Preferences window allows you to enable and configure the options for inserting/overlaying a PDF document onto an existing one.
The window can be opened by choosing the Printing Preferences menu item when novaPDF is selected in the Devices and Printers window and clicking the Overlay tab. This tab is also accessible from the printing Windows application in the Print dialog if you press the Properties (Preferences) button when novaPDF is selected.