No easy way. If you just go wider with the treads and the boards hang over, you could hang your toes as you go up from one to another. You might could add 1/2" and get by. If you needed to add much at all you'd have to tear out the whole stair case and lengthen it, adding more steps. Most cities have codes stairs must be. Big mess that cost lots of $, doubt you want them wider that bad.