• create a folder IMPORT in your notes-manager and drag it to the very beginning (so that it is now the first item; not necessary but one other step is then easier)

• open your external notes.adr in HEX-editor (or text editor capable of reading non-text files; eg. PSPad can do the job)
  (you should see both hexadecimal and text representation of the file side-by-side)

• select and delete its header, ie. from the beginning of file ("Opera Hotlist ...") to the first note / folder (see below)

• select and delete THRASH folder (usually at the end of file) (see below)

• select and copy all

• close Opera, go to your profile directory, find your current notes.adr file, make backup of notes.adr, open notes.adr file in editor

• navigate the cursor into the IMPORT folder (now it's fine if it's the first one; it has NAME=IMPORT attribute) right at / before the - (minus) representing the end-keyword of the folder (see below)

• paste, save, close notes.adr, run Opera, done.

remember:

• NOTE item is always indicated by #NOTE keyword at the beginning, it ends with 0D0A0D0A in hex-code which represent unprintable characters (exactly "double enter" at the end of line) - you probably see 4 dots / full-stops / weird characters instead ... and is followed by another note (#NOTE) / folder / (divider)

• FOLDER item begins with #FOLDER keyword, ends with 2D0D0A0D0A which is - (minus) and again those 4 characters and is again followed by another note / folder / (divider) / end of file

• THRASH FOLDER is usually at the end of file, it is a normal folder, ie. begins with #FOLDER keyword, it has THRASH FOLDER = YES attribute somewhere in the middle, it ends as a normal folder

I hope it helps someone.

Where exactly is that? I have gone to C:\Program Files\Opera, but I don't see it.

It's not there, of course.
Click on About Opera in the Help menu - you'll get a list of your browser stuff locations :idea: