I've never altered a hookie, but I have shirts and oneies. I made the front on the shirt be the back and cut down the back to make it smaller. I used a doggie shirt to go by and cut the belly up shorter. I cut the bottom out of the oneies and then did the same. It was fairly easy. For a hoodie, I belive if it was a close fit, I would just remove the zipper, lap it over and put velcro on it. Good luck! Show us what you come up with.
Here is a picture of Rily in one I did.