Your cells already know how to produce ATP from fat. (except for brain cells, and maybe a few others) there are more steps involved than the glucose pathway, but the cells already know them. All you have to do is up regulate those pathways in the cells that already have them. This is why your body makes/stores fat in the first place: it is a nice way to store a lot of energy for later use that doesn't have the problems of excess glucose in the body, if the body couldn't turn it into ATP it wouldn't bother making fat.

Ask a real biologist for details. I know this was covered in my high school classes 30 years ago, but I haven't studied it since so I don't remember details or know anything later research has discovered.