Controller digital to analog conversion?

[Megumin]

Atari 5200 has a reputation for shoddy and unreliable controller and its biggest fault is the analog stick is not self-centering and some of the games aren't designed well for analog controller like Pac Man and Pitfall!

I would like to use a more common digital controller (such as Atari 2600) instead of analog controller for some games but how would I go about that? The console has 2 input for analog controller, horizontal and vertical plus common. The voltage level determines the controller position. Digital controller works differently by shorting to ground on direction and floating high when none of the direction is pressed. How can I change that so for example, when I press up, the signal goes high. When it's in neutral position, the signal is middle level, and when I press down, the signal goes low?

I've looked around and the schematics I find are for PC analog controller to 5200, which would be easy but it's not digital adapter. There are adapter for digital controller like Sega controller to 5200 but schematic is not posted.

 

[dr pepper]

It would require some significant surgery.
If you connect the up button so it pulled the up/dn input hi (assuming hi = up), and the down button so it pulls the up/dn input low, then connect 3 resistors across power and ground, 2 fixed and one trimmer, say all 1k ohm, then connect the wiper of the trimmer to the up/dn input, then you can adjust the trimmer so that the console 'sees' the input as centered when the 'stick is in the rest position.
Obviously you'd need to do this for both the x and y axis of the 'stick.
Its possible to do this with an inline box so no mods are required, but that means more complexity and a couple of transistors to invert the polarity of the buttons.
Not too hard if your handy with a solder iron.