So what you're saying is, /var/home_volume already exists and has files/dirs inside. In addition, /home already exists and has files/dirs inside. And you want to merge these two directories so that all files/dirs from BOTH locations are shown side-by-side in /home ?
The cleanest way, I would think, would be to move the contents of /home to /var/home_volume, delete /home, and then symlink or bind mount /var/home_volume to /home. If that's not an option, I suppose you could loop through all files/dirs in /var/home_volume and symlink them inside of /home
for i in /var/home_volume/*; do ln -s $i /home/; done
But this would have to be repeated every time a new file/dir is added in /var/home_volume, it wouldn't be added to /home automatically. It would be pretty messy though, and how would you handle files with the same name in both locations? You'd be better off thinking of a new way to do things, IMO.
All of the above suggestions are explaining how to make a symlink of /var/home_volume INSIDE of /home, so you would have to go to /home/home_volume to see it. It doesn't sound like that's what you want.