i think you are confusing the fact of "can you do it" with "will you like it".
you seem to already know that it's possible to use your PSX pad.
just do what allnatural suggested, configure them how you like. the PSX has 12 buttons, 2 joysticks, and a dpad, the N64 only has 10 buttons, one joystick, and a dpad. you'll figure something out
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Dedicated to mediocrity and repetition since 1994.