Hasta La Motion!

I have been trying to find tutorials on the net using Zdepth to help composite images together. All I can find is how people use them to make Depth of Field. I am using a book called "The art of Digital Compositing". The book explains that you are able to use Z channels to help composite two images that have Z channels and blend them together. I have a render of trees and floating spheres. Each have their own Z channel and I would like to comp them together to have the balls appear withing the trees and not just on top.
Is this technique possible with After Effects or more preferably Toxic (Composite)