I've searched the forums for this specific question, and the answer so far seems to be that the wire I want to add (which doesn't define a hole) must have opposite orientation to that of the outer wire.
Well, I've been experimenting with it, and it doesn't really matter if it's forward or reversed, it makes the whole face "unfilled".
I think it's important to say that the wires I want to add that do NOT define holes are open wires. All closed wires I add are holes. The problem could be in it.
Is there a way to add these open wires to a face without making it invisible?
Thanks in advance.