How can I create one PCB that I can plug-in to two different places in a mother PCB?
This is not currently possible in KiCad - you can't create more than one PCB per project in a sensible way. The schematic file format is just text, so you could do some scripting to cross reference the interfaces. The usual way to do this would be to actually make the interface a specification of the design, and have an ECO raised if you change it. Obviously this is unwieldy for a personal (presumably) project.