It's going to be tough with just the volumetric clouds, because there aren't any good deformers for them. The shape of a mushroom cloud can be divided into a column, a hemisphere with an upward bulge on the flat side, and a torus for the edges that sink as the outer edges cool. You might be able to simulate those shapes with one cloud for the column, one with a flat bottom for the top, and several small ones spread out at the edges for the rim sinking.

The Primivol plugin from Inagoni would probably get you closer to what you want. I haven't used it myself, but I hear great things about it. You should be able to do your basic shape with a simple model, then fill it with the cloud texture of that plug in.

Another possibility would be to use a particle system. Make a polygon mesh for your basic shape, with an alpha mask that makes it completely invisible. Set your particle emitter in the mesh, and constrain the particles to stay inside the mesh. A wispy, smoky image on a smal splat primitive (such as you might use for a plain column of smoke) might get you in the general neighborhood. It might even make sense to have two emitters, one for the column, and one to spew out particles from the top of the column out to the edges.