[Mono]

So, it works if done exactly as tuxor suggested:

1. create a new scribble inside the scribble app and remember the name (e.g. scribble_13)
2. after that remove the file /root/notes/scribble_13.sketch
3. copy or move/rename the sketch file you like to import to /root/notes/scribble_13.sketch

that's it. the thumbnail will refresh next time you open scribble_13.

What does not work (at least in my case) is, if you skip 1) and just replace not working scrible file with good one. I do not know, why it does not work. Maybe that somewhere is stored another information about that scrible file and so not all information needed is corrected by just replacing scrible file....