Quba HQ saya he's working on a project but can't explain the details yet in Volumetric Clouds in After Effects? Yes We Can!
One way to do that in After Effects was mentioned by Brian Maffitt on an old Total Training tape. He demoed making a 3D volume of a human from scanned slices of some poor guy and said one might do the same with renders some Fractal Noise or similar as offset 3D layers.
There's probably a variety of additional techniques found through some searching, and maybe Andrew Kramer's cloud still as particle might help. One option might be to use the AE filter Aurora Sky: "Taking advantage of the new 3D capabilities of After Effects, Aurora Sky lets you use texture and cloud shaping maps to create an infinite variety of skyscapes... [it]... works in 3D space, both its own and After Effect’s. The built-in camera can fly throughout the vast world within the plug-in, using all the standard axis controls."
Update: as noted in comments, Aurora Sky might not work. QubaHQ never posted but the wireframes were rendered. There was a successful project from Dan Sollis though: