A simple way is to realise that a foot print is not necessarily a traditional 'component' like a connector or resistor. It is a geometric collection of pads (and some other graphical lines like edges, holes and silk screeny stuff). So consider the KiCad inbuilt Arduino shield 'component':-
In many ways it's similar to your BoB (4 separate connectors). And you have a well dimensioned pcb. So just create a new component called a BoB with your 10 connector pads. Use the footprint editor. And you can also use the symbol editor to create a new part symbol called BoB. You'd then use it like so:-
Just try to get your head around the fact that the schematic bits you'll be designing will be atop the physical footprint, yet draw away from it on the schematic sheet. This took me a while, but once understood, it's really handy as you'll be able to share both the footprint and symbol between the backplane and BoBs.