There's a volumetrics pass, but that's all volumetrics in one place and I think you are asking about those different volumetric objects appearing separately. But I think if you swap things around and use an INCLUDE list, then LightMix will do the trick - Corona Light only includes the volumetric object, then its LightMix will in effect be just that light's volumetric effects. If the light does have to illuminate other objects in the scene, you could duplicate it and have the duplicate exclude the volumetric object this time, then you have a LightSelect for the volumetrics only, and one for the non-volumetrics only. Not sure if that's what you're looking for though....