Not sure if it was mention before but a search turned up nothing.

This actually should have been included in the Enterprise uniform pack. I'm not sure if this would work however with the current available slots for clothing.

I agree. I liked the baseball caps on Star Trek Enterprise, although they hardly wore them.
IIRC there is no slot for headgear, but there are NPCs that wear hats. So I'm wondering how hard it would be to add them? I think you're right. It would mean reworking the costume slots. I don't think it will happen, but it would be cool if it did.

I was thinking it could be made part of the hair design options tab/slot. This could open up other head gear well. this idea is all low priority any way. Other things to be fixed first.

Hair design slot is an interesting option. I could see that as a possible work-around. Isn't there an option to turn stockings off for the TOS female uniforms? Maybe it could be like that. You get the hat on your head with the uniform top, but you can unselect it if you don't want it.
I agree that this is a very, very low priority- if one at all.

Why not just put it in the headgear slot with the visors?

I'd like to have ENT style hats if we could also have fleet emblems on them, or even be able to make our own ship emblems.

Headgear slot you say? Seriously, I forgot about that one. Yeah, I don't see why that shouldn't work. But I can see some wacky bug where the hair and hat get clipped together.

Or possibly put them in with the other head options like antenna and horns?
Hat's seem to fit better in that category (i.e. they're all things that go on top of your head, above your hair).

And while we're at it, what about the hats from the 1st TOS pilot (the one with Cpt. Pike)? Or the Niners ball caps from DS9?
http://memory-alpha.org/wiki/Hat

Pfft. I'm still waiting for them to give us a Headgear slot for our Trills and Vulcans and what not