Yes, there are schematics for the two "normal" controllers. This is for
each port with J common:
But you have other two commons repeated on each port, K and L. I don't know if somebody has tried six joysticks before...
As I said before, to avoid ghost pulses with more than two joysticks, every joystick port on every direction and fire lead must be provided with a diode. Current goes from the commons(J,K and L) to the directions and fire leads, so the diodes must have the strip(negative) towards A4, A5, A6, B5 and B6.
The Edge connectors are of 2,5mm between contacts if I remember well. You must cut and adapt yourself, without turning down them, as there is also 0v and +5v on some pins.
There is an easy way of reading that extra buttons, like the "normal joysticks". An Out with the row(0 to 9) to Z80 port B5h, and an In,a from port B6h. Bits = 0 to 2 give you information of the same button but from three joysticks at the same time...