It's nix, of course it's possible and always has been - just not integrated in to the WebGUI ....yet.
But creating additional FTP users from the shell is painless, just create new user mimicking the existing UID of the domain owner, home directory and add to psaserv group and presto - additional FTP user for same domain. As the UID and group is the same as the original domain owner - same rights.
Example:
# cat /etc/passwd | grep 'original_domain_user'
original_domain_user:x:10101:1234::/var/www/vhosts/original_domain.com:/usr/local/psa/bin/chrootsh
UID is 10101 (in this example)
/usr/sbin useradd -u 10101 -o -d /var/www/vhosts/original_domain.com -g psacln -s /usr/local/psa/bin/chrootsh new_user
passwd new_user