It can be done. NES and SNES has simlar protocol with SNES using second chip for 4 extra buttons and PCE uses 2 chips for extra 4 buttons as well. Remove the shift register IC that Nintendo used and figure out what button goes where, then put in PCE's IC and wire it up. If you want SNES pad to work like 6 button version (X, Y, L, and R), that requires 2 chips and some more wiring. Finally you'd need the PCE or TG-16 plug rather than SNES plug. If the pad you're hacking has rapid fire option, they often can be left in. When I hacked my NES Advantage I was able to keep original rapid fire option for PCE.
If I had a spare SNES pad I really liked, I would have tried it already. SNES Advantage doesn't come cheap at over $30 on scamBay and regular SNES pad isn't really made for PCE 6 button style gameplay (no rapid fire).