It seems more complicated this: open piano roll back, opens another piano roll, hit, set ... too much ... In selecting the notes you want to modify and change the volume on the same piano roll enough without further complication ... I think Me parece más complicado esto: abre piano roll copia, abre ...
I do this in spanish with video, sound and images:
Notas superpuestas en LMMS-Tuning overlap notes in LMMS
I feel this very interesting and work.