How can I sing with a rock-ier edge in my voice?

I almost hate to say it but practice, practice, practice. The more you sing in the style that you like, the better you will get. A good sound engineer with decent equipment can enhance your voice electronically to sound edgier than it does naturally, but your best bet is to develop that yourself. Work at it regularly, but don't push or strain. Yeah, stay away from the cigars and whiskey, they do more harm than good.