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.
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