Most of what I have seen along this has been postwork (seen a couple tutorials). I know there are a couple products that have a prop that is used to mimic volumetric lighting. That I know of there is not an easy way to do this in D/S. I believe its AurthorPendragon over at DAZ that has a product that lets you get to the options to adjust the falloffs of the lights in D/S but I have not played enough with them to say if they will do what you want.
G'luck