Hello guys,
I have a question about KNotes. I can add the a note by using the "Add Widget" function, which obviously is not convenient at all. So far I haven't found a way to add a new note if I already have a note in my desktop or in the taskbar. Is there any way to achieve that, or should I fill a bug report for it?

The "Sticky Notes" and KNotes are unfortunately independent. Adding a sticky note to the desktop doesn't make it available in KNotes, and vice versa.

OK, you are write, my mistake.
I am talking about the Notes Plasmoid.

This plasmoid needs some love. There are a lot of staff that missing:
[*]You cannot hide a note!
[*]You cannot ADD a new one, directly!
[*]You cannot retrieve older notes that are stored already in the ~/.local/share/plasma_notes/

So, here is my question for today:
I have a few important notes in the ~/.local/share/plasma_notes/. How do I make them visible in my desktop?

You may want to file a bug on bugs.kde.org for these problems you are experiencing (one per report).
I've moved this to the Plasma subforum so that other people might suggest workarounds and / or solutions (I don't use notes enough, unfortunately).