I've just completed a tool that create the tabs with mouse-bites between pcbs in pcbnew, it's on gitlab here:
https://gitlab.com/dren.dk/kicad-utilThe workflow is quite simple:
1: Start pcbnew outside of a project
2: Append and place completed boards.
3: Draw lines between the boards in the Eco1.User layer where you want tabs to connect the boards.
4: Run my kicad util with the panel option.
5: Open the newly created output pcb file and re-fill zones or run DRC or whatever else you need to do.
The tool also creates lines 3 and 6 mm away from the holes to indicate the distance that should be respected for tracks and smd caps respectively.