This will probably depend on your particular environment.
If you have Windows users then you can use the CIFS home dir functionality to set the FTP home directory. If a user logs in and ONTAP cannot find a home directory then the login is denied. If, however, you set ftpd.dir.override to a valid path and ftpd.dir.restriction is set to off then users without a home directory will have it set to the override path.
You can then simply create folders in the path specified in the /etc/cifs_homedir.cfg file for named users and everyone else, your generic users, will get directed to the override path. Obviously if you are already using the CIFS home dir functionality then this may conflict.
If you have local users then you would set this in the /etc/passwd file.