Accessory coat from outfit

I don't know if there are any tutorials out there specifically on this, but it's the same principle as making any accessory that doesn't fit a default category:

You would clone an accessory, possibly a ring.

You would separate the coat from the outfit, import it into your ring mesh and replace the ring with it.

You would move the UV map of the coat off of the body (since that is where proper clothing is mapped), probably into the "extra parts" slot.

That's the gist of it. There are a number of things you need to keep in mind though. First of all, you need to be selective in what you're cloning off of. Obviously if you want your sim to wear necklaces and earrings with your coat, you wouldn't want to clone off of them, since then your coat would override whatever it is you cloned off of. So most people clone rings. However, since most people clone rings, it means many non-default accessories share the slot, meaning one will override the other, so your coat may not be compatible with someone else's purse, or scarf and so on. If you plan to have a long-sleeved coat, perhaps you could clone a bracelet instead.

Same with the UV map space. Not only is it often occupied by existing clothing (many sleeves, ruffles and other elaborate parts are mapped there), it is also where other people map their own non-default accessories, so once again your coat may not be compatible with a lot of custom stuff since you'd end up with one item's texture bleeding onto the other.