It'll only read mp3 files. m4a is a completely different format, so you can't just change the extension.
click on the iTunes menu
on the [General] tab, click [Import Settings...]
for [import using] select [mp3 encoder]
click [ok] again
Now when you right click on a track in iTunes, about 2/3 of the way down the list there'll be an option to [Create mp3 version]
Once you've got mp3 versions of the tracks you want, just copy the mp3 files into folders on the SD card.