Zoom texture on sprite

Hi everybody,
i implemented a cloud layer using a sprite. I use setTextureRect() for animating the clouds. It works fine. But now i want to zoom in and out. Normally i think i just have to multiply/change the texture coords. But i do not have access to modify them directly and i did not found a way to modify them correct using setTextureRect().

Is there any way to do it?

Thanks!