Stolen from http://kb.parallels.com/415
Is it possible to create FTP accounts to go to a certain directory on the server that is not your_domain/your_name/? We need to be able to create FTP accounts that only go to a specified directory.
This feature is planned to be implemented in the future Plesk versions. This time it can only be done manually.
Login to the server as ‘root’ user and create a system user (for example USER_NAME) with home directory you need, for example:
# /usr/sbin/useradd -d $HTTPD_VHOSTS_D/your_domain/some/location -s /bin/false USER_NAME
Set the password for this user using ‘passwd’ command.
Add this user into ‘psacln’ group:
# /usr/sbin/usermod -G psacln USER_NAME
The user will be ‘chroot’-ed to his home because ‘psacln’ group is listed in /etc/ftpchroot. Set read/write permissions for this user on his home directory, for example:
# chmod 755 $HTTPD_VHOSTS_D/your_domain/some/location
# chown USER_NAME:psacln $HTTPD_VHOSTS_D/your_domain/some/location
And set execute permission on the domain’s httpdocs folder:
# chmod 751 $HTTPD_VHOSTS_D/your_domain/httpdocs
$HTTPD_VHOSTS_D – have to be replaced with the real path to the Plesk virtual hosts in the all examples above.