Unless the hat is modeled that way there is no current way to mix hats.
I may be able to mix those hats for you.
He's not asking about equipping multiple hats at once I believe, but having the scarf hat and the default player helmet on at once.
I haven't messed with this mod so I don't know how it'd look or if this is what he is talking about.
But it should be possible, just need to find a line that hides the helmet node on the player and comment it out.
But that'd make it so the helmet node will always stay visible.
