How do I import one PCB into another one with Kicad?

Open your primary PCB and select File -> Append Board Choose the *.kicad_pcb file you want to import. This will be imported as a group. If you want to ungroup the imported PCB (snippet), then right click to bring up the context sensitive menu and select Grouping -> Ungroup Items. For the imported snippet, you'll probably want to make sure any footprints are unlocked first.