Transcoding for audio playback device. How to?

I have an audio library with a lot of aac(m4a) and mp3 files. Is it possible to create transcoding profile for sent mono (sum of all channels from source file) mp3 stream to device?

[zip]

No, you can force stereo, but not mono.

Is it possible to transcode to joint-stereo mp3? If "yes" what should be added to profiles.xml?

[zip]

No, it's not.