One thing you could try is just using a sphere with a volumetric material. (Though it takes forever to render) I tried an experiment with the Bryce "Volumetric Ash" material that looked passable. Animate the colors and density of the material. Also place a spotlight to shine on the centere of the cloud with lots of intensity & diffusion. Animate the intensity and color of the spotlight. Still, all of this take a LOT of time to render. Does the explosion absoultely HAVE to be done in Bryce? Particle Illusion SE or AlamDV are a couple programs that can add explosions to video clips. PI is my favorite, but the both sell for under $100 and make it fast and easy to add explosions. Just something to consider. Good luck.