It would be very handy if it is possible to input a second voice with Lock To Duration Enabled. The second voice follow the rhythm of the first voice. It could be set off when the second voice has another rhythm.
You can achieve this result already in two steps by copying the existing music, using Edit > Paste Into Voice to paste into a new voice, then using Lock to Duration as you repitch the pasted notes in the new voice.
Great tip Daniel, thanks!