physics - Box 2D Set Origin

I'm trying to set the origin of a box2d shape. By default, a shape rotates around the center. But I'm trying to get it to rotate around the left of the shape. Like a clock for example.

I've heard of things like SetXForm but that function doesn't seem to even exist. I'm using C++ just in case you needed to know that.

Thanks in advanced, any help is appreciated.