When building a flight simulator cockpit, many switches at various positions replace the keyboard strokes. If you still want to use the keyboard as interface to PC, the cockpit switches need to be wired back to the keyboard.
Mechanically wise, there are two kinds of keyboards: The more expensive types still have a PCB (printed circuit board) on which real switches and electronics are mounted. The cheaper keyboards use layers of plastic foil with copper patterns and carbon-in-rubber press switches. The foils connect to a small PCB with the controller IC.
If you want to connect many external switches to a keyboard, by all means try to find one of the keyboards with PCB. Older keyboardsare often PCB based and can be found at your local junkyard / recycling place. (You may need an adapter from the old DIN type plug to the newer PS2 type plug) Also many DELL system keyboards seem to have PCB with switches.
Most keyboards have about 100 switches, which are wired in a matrix to a keyboard controller IC. This means that the keys are floating, i.e. they don’t have any common ground point. If you want to keep your setup flexible, the easiest way is to solder a little connector across each keyboard switch and connect your external switches parallel over the keyboard switch. You’ll need two wires for each switch, and the external switches must be isolated from each other. The circuit and picture below shows the idea.
Note: These switches must be normally open. A continuously closed contact will produce a key-repeat.