I think cutting off the USB plug and soldering on an Atari plug would make it a lot harder for someone to plug into their PC, and even if it was wired right to fit an Amiga, the best you'd get out of it is the use of 2 buttons without a PIC chip converting the signals to CD32-compatible ones. But then you'd need to learn the exact workings of that partiular pad and its button layout to do that, which is why no one has made any USB-CD32/Amiga adapters yet. If you were wanting to use a new controller on an Amiga there's no better option than getting or making a PS2-CD32 adapter and buying a PS2-compatible controller, which are just as easy to find, if not easier, than USB ones, and they all stick to the same standard format/button layout unlike USB so they should all work without drivers or manual configuration.
If it's for the PC, then Photon's method should work well and save you a few bob, but don't cut off the USB plug if you're going to be using it in a PC, or if you want to retain the use of more than 2 buttons. There are a few local Australian places selling wholesale arcade machine parts, so you may actually have a choie between all three different stick types: Japanese Ball-top, American Bat/club-top or Australian-style plastic ballstick.