If I understand you right you want to chop of the psx controller and wire the cable into the snes controler? That would be imposible. Maybe you could Remove most of the psx controler and put what you have left into the snes controler. It would be very hard if not imposible. You would need to find some way to keep the control chip, and the resisters etc. from the ps2 controller.
I know there are some ps2 controlers that didn't cover the pins of that chip with all that black crap so you could see were all the buttons traced back to. It was one of the old ps2 dule force 2 controllers. I used one of them I had laying around the house when I made my arcade controll panel. The new ones have the black crap and 2 seperate grounds, found that out the hard way, one for start and selct and one for the rest of the controller.
If you can find one of the old ones you could in therory remove the control chip etc. and solder everything up dirctly to that chip; then put the chip inside the snes controler. But like I said this would be very hard if not impossible. Its been a long time since I opend up a controller so I don't know exactly what would need to be kept.
Edit: Digital controllers my not have the black stuff on them, I have never used a digital one only the analog ones.