Using M4a format with Ipod nano

Hi,
I grabbed my own CD's with Itunes and created m4a format music files which I can read with any music player. It seems however that this does not work with my Nano.
is it correct?
is there an easy way to transform the M4a back into mp3 without regrabbing all CD's?

Baf

Fujitsu-Siemens Lifebook M Series Windows XP