I doubt if there's a way of doing this, but just checking.......

I would like to be able to take a single line/voice, and reduce it to a monotone, without having to treat every single note individually in order to do so.

(Why do I want to do this ? To make a practice tool for improvisation. If I could take a jazz line, and reduce it to its rhythm, as a stave notation on one note, and I then applied a chord progression to it, I could practice improvising, able to concentrate on the note (= pitch) choices because the rhythm was 'given'.
Just an interim measure, but I think it could be useful.)

I suppose a work-round would be to have Composer play the original line, and record against it from keyboard a second line on one note .