am I supposed to edit recalbox.conf in share/system ?
Yes.
Let's say I want the 'ports' to be listed on the 10th place, where exactly do I need to add a new line or do I need to edit an existing one ?
You will need to add this line, it does not exist.
Open recalbox.conf and search for "Arcade metasystem", you will see that this option is commented out:
; emulationstation.arcade.position = 0
I would add just below that line, or, at the end of the existing lines in "Virtual systems" (which is just below "Arcade metasystem").
If I'm not mistaken, there is no option to define the exact position, you can define it at the beginning or at the end, but, I never tested it, as I don't have this need.
Test it, see what you get, and share it here.