Interesting suggestion. In general, I would say No, but in some cases it could be useful. For example:
A reply node A is posted twice (accidental duplicate B), and the second reply (B) is considered for reaping. But B also has answers, and consequently receives upvotes. The end result is that B has, say, 15 votes to reap (and none to keep or edit) but non-negative rep, so it can’t be reaped automatically. In this case it would be nice if a janitor could transfer B’s upvotes (if any) to A, re-parent replies to B as replies to A, and then reap B. But I doubt that such a fix, even if generally approved, would be practicable to implement.
If janitors ever do get the power to reap nodes manually, I think there should be at least two preconditions: (1) the node has a high number of reap votes (significantly higher than the current threshold for automatic reaping); and (2) at least 24 hours should have elapsed since the node was posted. But of course that’s just off the top of my head. No doubt there are other factors I haven’t considered that would need to be addressed before such a change could be contemplated seriously.