How can I hang a shelf off of existing screws in brick wall?
I wouldn't use those screws since you have no way of knowing how they were installed.  However, I would suggest you look into brick hanging clips (use Google to find them). These are made of metal and are designed to grip the bricks. No holes are required. You made need a number of them depending on the weight of your stuff. I have used them to hang pictures on brick walls.