I think I did not explain my problem well :/
I'm a beginner, sorry if I use the wrong words.
I'll try to explain you my problem clearly this time :
I created a new circlet mesh, I wanted it to replace the already existing female helmet mesh (the Daedric Helmet for example), so I modified the "biped object" into a circlet, then I modified the model's (DaedricHelmetAA) "biped object" into a circlet.
There comes my problem :
Now the game wants the female *and* male helmet to be a circlet, so I can't see male helmets because the "partition type" (in OutfitStudio) of it's mesh isn't a circlet. However, I can now see the circlet I created on females.
Sorry for the last time. I was not clear at all.