After wandering around with grid and referencing point I need to come back home.
In Footprint editor I just want to place the grid origin on the axis origin (the violet small cross).
I’m not able to do this!
Since that point is not on the current grid I simply cannot select that point! Of course I can press CTRL and disable grid snapping, but then I rely on an “optical alignment”:
Is there a way to force snapping on that special point?
Change the grid to something finer.
Thanks, but I already said in the question I’m able to do this. It’s a workaround and not precise.
Anyway, from your hint I should guess: “the answer is no, you can’t snap on the origin”, am I wrong? 
Why do you want to place the origin off grid?
It’s the other way around: I want to place the grid on the origin!
Ah, sorry.
This is about placing the footprint anchor and the Grid origin on the same position.
You need to move the anchor to the grid.
The anchor is the icon above the grid origin icon. Click on that then place your cursor on the grid origin and click.
Is there a footprint that I cannot see that surrounds your screen grab?
Yes, the footprint is almost done:
I’ve just tried and perhaps I’m missing something because it moves the whole footprint (ok, I’m actually changing its starting point!) but the violet cross (sorry I didn’t find the correct name in the docs) does not move.
I just want to place the grid origin on the axis origin (the violet small cross).
Is there a way to force snapping on that special point?
main menu bar → Place → Reset grid origin
Sorry, was called away.
The violet cross is the footprint anchor or centroid.
This is the place the footprint is attached to your cursor when you place the footprint on your PCB.
The anchor is usually placed last when creating a footprint.
In your case, you probably want it in the centre.
The grid origin (white cross) serves no purpose for a completed footprint and can be ignored.
Yes, got it. I’m afraid I was not able to explain what I was trying to do. I apologize for that. The above answer is correct: Place → Reset grid origin just puts the grid origin on the axis origin.
I need to investigate further, since you say: “The violet cross is the footprint anchor” but using the footprint anchor command does not move the violet cross. I’m going to read again the documentation 
And I was totally confused; wondering what an anchor was doing on an empty screen. 
