It looks like you are re-texturing the long sleeve police uniform(my pics
assume this). This should help with your hat.
If you have already made the hat then this should help:

If you click on appearance and look in the costume element box, you should see what type of hat your costume has. This is important to help you place your new hat.
If you go into the
extract meshes wizard, scroll through until you reach hat_pol. This will list all the hat_pol versions you have in your game. See below:

In my case my hat_pol goes up to 14. So I would save your new hat as 15. That should make it seletable with your uniform.