Hm, I tried this and as StefanF said, hitting Q did not change the tuplets. Anyways, besides adding the new notes to the existing one (how to delete the existing rhythm note afterwards?), it does not work as soon as it comes to dotted quarter-notes which should be converted to normal ones.
The lock function does not seem to work with pasting, but this (being able to paste rhythm and pitch independently from each other) could be a quite powerful feature, not only for this workaround but for other work as well. Maybe something for one of the upcoming updates?
Seeing this strange output, I wonder: Does Doricos engine handle tuplets with their actual length as well or not? It handles tied notes with their actual values, so why not (it looks this way to me) tuplet notes?