Napster to iTunes?

How do you transfer tracks from napster to iTunes? I need to put my tracks from napster onto my iPod, but only the ones i have purchased are showing up in my itunes library. How do I transfer them, for free? I have the iPod nano chromatic 8gb. Whenever I try to get the tracks (unbought) from the "music" folder on my computer, it says that it was not copied because it cannot be played on my iPod. Please help, it's my first iPod, and I realy don't want to pay around $1,000. So my overall question is: *How do I transfer unbought tracks from napster onto my iPod/iTunes without buying all the tracks?*