Depends on the mesh. If it's a separate part of the hair mesh, it's usually pretty easy, by deleting the hair groups or manually deleting the polys. But if it's shaped to the hair or "glued" to the base hair mesh and have a big gaping hole you'll have to cover up, then it might be a bit more difficult. It is possible, if you're patient and know a bit about meshing.
If you don't know a thing about meshing, I'd suggest trying objects first, then move over to bodymeshing and accessories when you're comfortable with it. Bodymeshing and accessories often involves loads of trial and failure, and loads of testing and tweaking, especially if you're new to it.
You can make hats much the same way you make other accessories. You'll have to take into account that some hats won't fit to most hairstyles, so putting the hat too close to the scalp isn't always the best idea if you want there to be hair underneath.
There are a few good accessory tutorials at MTS, and probably some here, by Bloom.
And might I suggest all-age versions, while you're at it?
For all-age versions, I tend to "clone" the shrek ear mesh at MTS. You can also use the all-age animal accessories, also MTS. Those are great, since they're toddler to elder, both genders, flashing blue accessories is already fixed, and so is all those little bugs that cloning basegame glasses creates. I've got Pets, so I know how annoying those bugs are...