Siggen, may I clarify your intentions? I'm trying to figure something similar out. If you were to do as you described, the additional stick would control the Dpad so that the X^O[] button functionality would remain the same. That way you could use your right thumb to toggle other functions with the X^O[]. I'm going to break this down just to be clear.
2nd Nub controls the Dpad and vice versa
X^O[] controls their original functions, X^O[]
Orignial PSP Nub controls its original directional functions.
This way in first person shooters etc you dont have to quit running foward to toggle the dpad. Your dpad and toggle functions (X^O[]) would be set up similar to the PS3 and Xbox dual sticks. Where you control the view direction and toggle functions with your right thumb.
If the 2nd Nub was connected to the X^O[] button leads then you have to use the dpad to toggle change weapon, reload, action, etc.
Do I have this right or am I way off base?