MP3 Maker doesn't currently support M4A as an output format, if that was your question. It's only supported as an import format, so you can convert it to other formats. You can encode to the AAC codec in MP3 Maker, but not in the M4A file format.
What are you trying to convert for? It's pretty safe to say that any device or software that does support M4A playback will also support MP3 playback. So, that might be your best alternative.