As we know we can add folder to user using
useradd user -p user_passwd -d /home/ftp/user_dir/ -s /bin/false but how to add folder to existing user, for example /home/ftp/root for root ?
Basically you don't add, you change home directory.
usermod -d /home/ftp/root root
if you want to move existing files, use this:
usermod -d /home/ftp/root -m root
Allowing root to access via FTP it not good practice, it's security hole.
Even if this, I would rather recommend to create symlink to target folder from existing directory.