There is an unfortunate problem with that particular idea. Apple cryptographically protects the AirPort Express audio stream to prevent exactly what you describe. Trying to crack something is definitely not a good way to cut your teeth programming, either ethically or in terms of relative difficulty, and I'm fairly certain the rules of the forum (not to mention U.S. federal law) would prohibit us from helping you towards that end.
Such a crack was already accomplished in 2004, shortly after the release of the AirPort Express, by "DVD Jon," the guy who originally cracked CSS. I really don't know the current status of that crack or any software that exploits it. It sort of dropped off the radar, probably because AirPort Express just isn't an especially inspired product. People tend to find it easier and cheaper to simply buy a third-party wireless router and streaming music box separately and keep all their music in non-protected format.
As an aside, how come nobody's answer to toddburch's question is ever a straightforward application that a neophyte programmer could be expected to work towards in finite time? For some reason everybody's itch is for something that would range from challenging to impossible a seasoned professional, like writing a driver from scratch for some piece of hardware otherwise only supported on Windows.
Don't get me wrong, I'm all for ambition and I'm not really criticizing, but it always strikes me funny the unerring accuracy with which everybody picks a first goal that would be guaranteed to drive them screaming from the profession in frustration.
Edit: Now that I think about it, I should probably mention that you can do what you just described legally (and semi-satisfactorily) without any programming at all by looking for software that will capture an audio input and pump it back out as an MP3 stream, then subscribing to the resulting stream in iTunes and directing it to the AirPort Express. It's a hacky solution, the quality of the audio source will be degraded and the resulting lag means you won't ever want to use it for real-time audio, but it is at least nominally possible to accomplish what you asked for.
