From what i have read on the net a ZR steering wheel will not fit a ZS car but a ZS steering wheel can be converted to fit a ZR. Not sure if this is actualy true?.
i am sure there is a link in in the how to section on fitting a zr wheel to a zs. i fitted a mg tf wheel to mine but you need to make a spacer its pretty straight foward to make. you also need to modify airbag aswell and it doesn't fit perfectly into the zr wheel.