I'd have to trial and error myself...
maybe you have to use an external script to parse the message and forward it to zensyslog, like in http://www.rsyslog.com/doc/omprog.html .
syslog-ng offers https://www.balabit.com/sites/default/files/documents/syslog-ng-ose-3.3-guides/en/syslog-ng-ose-v3.3-guide-admin-en/html/rewrite-replace.html