M4As are supported, as long as they don't have DRM on the file.
If you want to convert your tracks within iTunes, you can easily do this by first changing your encoding settings (iTunes preferences > General tab > click the Import Settings button), change the encoder to MP3 Encoder, and be sure to set the bitrate values high enough (and non-variable). Then you can select tracks within iTunes and right-click to choose "Create MP3 version."