What do you mean exactly? Do you mean connecting an 8-way stick (like a Sanwa JLF or Happ Competition) to the analog points on a controller instead of the d-pad (kind of like the LS option on Madcatz sticks)?
Or do you mean using an actual analog stick instead of the normal 8-way stick?
If it’s the first one then I don’t think it would be hard (but I’ve never done it). The second one I’m not so sure, there would be no use of an ADC though because you would be connecting an analog stick to the analog points on the controller. The d-pad can only handle 8 way I would guess, meaning 000 to 111 in binary and it wouldn’t be able to handle more bits (more directions).
For the first one what is the reason you want to do that? So that it will work in more games? If you plan on using it only in fighting games then it would be easier just to use the d-pad. If you’re really set on having the 8-way stick control the analog portion rather than the digital you do not need an ADC.
The analog stick is merely a variable resistor so (I think) you need to find the appropriate values for Up, Down, Left, Right and use resistors with those values on the connection from your stick to the pad.
Hopefully someone can come and confirm this.