Uhm. Well, the nunchuk uses 2 potentiometers to output 4 axis'. Y+, Y-, X+, X-.
The PSP analog stick outputs Y and X values as well, but in a different format.
It's possible...but you'd more or less need a microcontroller or PIC to interpret the X/Y from the PSP analog stick and then output corrected values to the nunchuk analog stick pins.
Maybe. Or, you could *attempt* to directly wire it by just reversing the schematic for the PS2/GCN/etc analog stick to PSP analog stick that Ben Heck made.
It could work. You'd have to test it.