Got it working
Code:
sudo /opt/zimbra/bin/zmcertmgr createcsr self -new -subject "/C=UK/ST=N\/A/L=N\/A/O=<company>/OU=<company>/CN=<domain>" -subjectAltNames "<alt-domain1>,<alt-domain2>"
sudo /opt/zimbra/bin/zmcertmgr createcrt -days 365
sudo /opt/zimbra/bin/zmcertmgr deploycrt self