When I first learned about XP, I was working as a system administrator and contributing to open source projects in my spare time. Those are the same questions I had.
I'm putting together a followup article entitled "Five Things Open Source Developers Should Learn from Extreme Programming" that will go into more detail. It should be ready in about three weeks.
For now, my best advice is to wear two hats. One hat is for the customer. Look at things from a user's point of view. The other hat is the normal developer hat.
It can be tricky to combine these responsibilities, but if you have the discipline to follow the other XP practices, you'll be able to do this one.
Thanks for the comments!