Get your ironport to add a new X-Header field when it considers the mail is spam, such as "X-My-Spam-Flag = YES".
Then two other steps are needed.
First one is to get all spams delivered to ZCS' spam folder (no spam discarded). In order to get that, you have to change one line in /opt/zimbra/conf/amavis.conf.in (from D_DISCARD to D_PASS) :
Second one is to add a high score to the mails with the X-Header you've defined. That's in /opt/zimbra/conf/salocal.cf.in, you add three lines :
#$final_spam_destiny = D_DISCARD;
$final_spam_destiny = D_PASS;
These two mods won't survive an upgrade, you'll have to do them after each upgrade.
header SPAM_FRONT X-My-Spam-Flag =~ /^YES$/
describe SPAM_FRONT Spam detected by our front filters
score SPAM_FRONT 50