That might be tough...iPhone ringtones are in m4a format, just renamed to m4r file types. Not a lot of programs *read* m4a format...
One roundabout way of doing it would be to take the files (usually in Music/iTunes/Ringtones) and rename them all to .m4a files. Import them into iTunes in a playlist (so you can find them all easily) and then right-click each one and select "Convert to MP3". That's about the only way I know of converting them back...
Audacity is the thing I usually use to *save* them as .m4a, but it doesn't *read* them at all. Even if you select "Import as RAW" it comes in as complete static.