No way man, do you know how does a USB device works?? In layman therms, a USB device comunicates with the PC (1st problem), identifies itself (2nd problem), asks how much power does it requiere and what is going to do (3rd problem). Also the computer should recognize your device, know how it works and communicate the received signal to the soundcard (4th problem). Solving all those issues would be a really cool project, but... only to connect a discman to a radio?
I suggest you to think in another simple USB kit to build, while you listen music with the iPod connection on your radio