Well basically you need to take an existing radio and measure the voltage
with repect to ground of the center wire on each pot at various throw positions.
As you move the sticks the voltage will change.

That voltage change is what you need to know in order to simultate the joystick pot
from the computer.

You can get a D to A chip which basically you put into the memory map of your
computer so that you can address it and write data (numbers) to it. Usually the
bigger number you write the greater the voltage on the output pin. Once you get
that working it will be a simple interfacing job to connect the output to the middle
wire instead of your pot.

Get yourself a basic book on micro processor electronics to see how to interface
the chip. You can use a latch and the printer port if you don't want to interface directly
into the computers memory address space. It's not hard but you'll need to do a bit
of investigation to design the circuit.