I have a scene exported from another package and I've created lights to represent the lighting from the previous package. I'd like to render volumetrics in Mantra and simply comp the shadows and emission generated from the smomke and fire. To put it another way I need to create a separate alpha for the smoke even though it's in a environment, I need to capture the light bleed onto the walls, and I need to capture any shadows from the smoke that might hit the floor. I'd render it all together in Arnold but Mantra is just simply faster.