As Lowlander said, you can convert as many files at one session as you want. Highlight all the files and click on tools\convert audio files.
To highlight bulk files, click on the first in the list then the last with shift key pressed. Keep in mind converting a lot of files at once does take some cpu power so you may want to stop playback while your doing this.
I think lowlander is also right about the plugins to do m4a may well need to be purchased, I have got them, not sure where or when, but I can convert mfa to mp3 anyway. You can do a search on google for free m4a - mp3 converters and do this outside MM, after which you can import the converted files into your library.
MediaMonkey 126.96.36.1994 Gold Lifetime
Hardware: Intel Core 2 Quad 3.33GHz, 8Gb Ram, 2tB Internal Storage, 2tB External Storage (USB & eSATA)
Software: Windows 7 Ultimate x64, FireFox v3.6.x, ThunderBird v3 x64, MailWasher Pro v6.5, Kaspersky Internet Security 2010
Backups by Karens Replicator v3.5.12,