Convert mp3 from mono to stereo using Lame
The answer to my own question is: No - it is not possible with lame
My work-around solution is to use the command-line tool sox and do the channel duplication prior to mp3-conversion:
sox mono.wav -c 2 stereo.wav
Then afterwards use lame:
lame -m j stereo.wav stereo.mp3
You can do that with ffmpeg. Here is the command I used for mono mp3 files with 11025 Hz sampling rate and 40 kbps bitrate:
ffmpeg -i mono.mp3 -ac 2 -ab 96000 -ar 22050 stereo.mp3