I assume the distance render is a seperate, grayscale image? Assuming they are the same size, open the Channel palette for the color image, click on the new channel button at the bottom of the palette (3rd button), Go to the Distance image, select all, copy and paste into this channel. The fist button at the bottom of the channel palette then loads the channel as a selection. The final step would probably be use Filter, Blur, Gausian Blur to blur an amount that looked correct.