tmonkey, the .m4p extension is for DRM protected files, so you should probably use a .m4a file instead. If you convert MP3 audio to AAC in a .m4a file, it should be playable by almost anything that supports AAC, including iTunes, WMP, and an iPod.
Try these instructions from Apple in iTunes and let me know if it works for you:
Both iTunes and the iPod support playing MP3 files so you don't need to convert them to use the files.
Also due to MP3 being a lossy format you will lose audio quality by converting it to MP4.
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