Thank you so much, Fladin. It finally worked.
However, there seems to be another problem. The helmets with mail coifs seem to not appear in the game.
It actually causes a graphical bug when I implement it.
My troops whom I equipped this helmet with are now bald XD.
I don't know what causes the problem, to be honest.
I've put everything where it should fit in. Maybe there's a different shader or texture for coifs in the mod I want to transfer items to.