useradd -d custom_home_dir_path username
would link the user and intended home directory at the time of the user account creation...
usermod could help if you don't want to/can't delete and recreate the new user account.
usermod -d new_home_dir_path existing_username
would change it, I think.
Perhaps one better:
usermod -d -m new_home_dir_path existing_username
should both change the user home directory assignment _and_ move the current/old home directory contents for the user to the new location.