mp3's on your hard drive to an ipod nano

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I got asked a question today about an ipod and i havent got one and ive never used one so i cant answer it. basically a friend of mine has mp3 files on her hard drive and she wants to transfer them onto her ipod nano, i heard somewhere about itunes not being able to do this, i heard the only thing itunes will allow you to do is transfer files from a cd/dvd to the ipod or files you have downloaded from itunes website. could someone help me out here please as i dont have a clue. Thanks in advance.