While version 1.4 is being prepared for publication, you can export the resulting audio to the AudioShare.
AudioShare's tools (top right corner) can convert to mono (as on screenshot).
The converted audio track is sent back to the LF project.
That is, you must completely finish editing clips with transitions on the timeline, but before adding other audio tracks.
After receiving the audio track from AudioShare, sounding in both channels, placed below the timeline, you can minimize the main track sound by the mixer on the left.
Thanks a lot and sorry for the late reply. Works just fine even if it is a bit tricky.