KiCad question: how to assign pins to pads when the footprint and device have different numbers of pins?
You need to design either:


your own footprint with the pin numbers corresponding to the existing tube symbol,
or your own library symbol for the tube, with the pin numbers corresponding to the existing footprint.


This means create your own footprint library, or your own schematic component library. That's a process with lots of clicks, but not actually difficult (you can easily create new footprints/parts from existing ones).