Anyway, here's an explanation as to why this isn't possible:
https://www.mobileread.com/forums/sh...d.php?t=339476
(post #6)
The other day I needed to merge highlights in a rare situation where I was reading in another system location the same book I had already in the library, then I ended up having highlights in two different files.
It would be so much easier if it was possible to copy/paste multiple json nodes at once using a Json editor like
this one, however in this case I had to open in a text editor the Json contained in %appdata%\calibre\viewer\annots and manually append the stray highlights I had exported previously.
edit: actually it was a duplicate in my library.