Ender6

If the FCS are previously Mac or PC dont matter because none of this original circuits/connectors are used.

In that conversions you are assembly a "new USB" stick with old FCS parts and potentiomters (or not if opt for use HALL sensor).

In that case are used a Diode in ground pin on PRO Micro board.

This ground pin are wired to all HAT and buttons:

http://simhq.com/forum/files/usergals/2014/11/full-38364-90920-img_2272.jpg


Diode are used to avoid "ghost"* buttons - a "collateral effect" in matrix wiring.
Since FCS has few buttons (4) only one diode are sufficient.

If the joystick have many buttons maybe will be necessary use one diode in each button or HAT connection.

* Matrix ghosting effect appear if more than 3 buttons are pressed simultaneous:
http://www.dribin.org/dave/keyboard/one_html/