That's actually not really possible, yet. I know I'm sounding fuzzy, however the concept of a "hero cloud" has not yet been implemented as such. There are ways, though, to create a mask for the density fractal, based on an image map. However, this concept bears a few issues for the renderer (strange lines in the clouds), and making the image map is tricky, too.
What you could try is to create an image map, all black and the place for the hero cloud is white. You will use this as a masking blend shader (xz projection with the right scale and placement) for a density shader, which in turn only creates color in the white region. So far as the theory. I have tried that once and had issues, but I don't exactly remember which. Maybe you're more successful.
Regards,
Frank