theThe '-R' usermod-R' usermod option does not change the root dir for a user, it changes the root dir 'usermod'usermod operates inside of, so you can use it to modify users inside chroot environmentsmodify users inside chroot environments.
Edit: I assume what you want to do is have the user placed inside a chroot environment when he logs in over SSH, you can use the 'ChrootDirectory' sshd_config'ChrootDirectory' sshd_config option to do that.