I think this is not really OP's request, but I tried to use a symbolic link to relocate some portion of documents on iCloud drive to an alternate location. I couldn't get it to work. Since I tested it, I just thought I'd report my results.
If you keep your Documents and Desktop on iCloud Drive, then ~/Library/Mobile Documents/com~apple~CloudDocs will have two symbolic links to your real ~/Documents and ~/Desktop. I created a new symbolic link there to another folder (the intention would be to link to a folder on an external drive). The symbolic link showed up on another Mac, but the contents did not synchronize. Also, the iCloud website did not show that directory.
I really didn't expect it to work.