How to use iCloud as a stacks folder?
This may achieve what you're looking for: https://discussions.apple.com/thread/6605847?answerId=26864942022#26864942022

Copy and paste from the above link:


  It's pretty much impossible to get the complete iCloud Drive folder in
  your dock. I did, however, find out how to add just the folders from
  iCloud drive that I manually added. It's a bit of a workaround, but
  this is how I did it.
  - In finder you have to use go to folder (CMD+SHIFT+G) and navigate to ~/Library/
  - Scroll down to Mobile Documents (This folders seems empty, but actually all your iCloud folders are hidden here)
  - Drag Mobile Documents to your dock (Once added to your dock you can see it's contents).
  - Look for the iCloud Drive folder via your new dock folder and drag this one to your dock again (yes it's possible to drag a subfolder,
  opened via dock, back to your dock)
  - Now you have a Folder called iCloud Drive in your dock. Remember this only contains the folders you manually added to iCloud drive, it
  doesn't show the standard folders. So this solution might not be
  good/work for everyone.
  
  As far as I understand, it's not possible to add your complete iCloud
  drive folder to your dock. The reason for this is that it's contents
  are actually fragmented in the ~/Library/Mobile Documents/ folder.
  
  Ow and don't play around (e.g. delete) any of the folders you find
  under Mobile Documents. This will damage your iCloud Drive
  functionality. That's a situation you don't want!