KiCad: PCB to footprint?
KiCad does not provide an out of the box feature to do what you want.

One option that comes to mind would be to use a template instead of a footprint. That would have the benefit that you have one entry in the position file for every connector needed instead of having only one for the full model. (Relevant for automated production.)

Another option i can think of is to export the board to freecad and convert it to a footprint in this program. (KiCad stepup would be the way to go with that. Showcase of the footprint generation feature)

You can also look for clues over at the forum. This thread might be a start: https://forum.kicad.info/t/converting-board-to-footprint/2781