Use some script with selection from database and /usr/local/psa/bin/mail utility. For example:
for i in `mysql -uadmin -p\`cat /etc/psa/.psa.shadow\` psa -Ns -e "select name from domains"`; do /usr/local/psa/bin/mail -c root@$i -mailbox false;done
but you should use it only as template for own script.