Adding Raspberry Pi Hat edge-cuts and holes in Kicad (after schematic capture)
Open pcbnew as a stand-alone application and load your completed board file.

Then, from the file menu, select "Append Board"



Navigate to the location where your templates are stored.  For me, that is /usr/local/share/kicad/template/ but it may be different on your system.  You can find where by looking at the path in the "Preferences -> Configure Paths" menu.



Select and open the raspberrypi_hat.kicad_pcb file.

This will import everything from the file.  You will want to delete the extra components.  Align your existing design with the connector and you should be set.