I know this is quite old - but I was looking for a way to do this and found the following way worked:
Use zentyal to create a certificate with the name you want.
Log into your zentyal server and replace the CSR found in /var/lib/zentyal/CA/reqs/ with the one you want to make the cert for.
Renew the certificate in Zentyal (this will use your new CSR)
Download the certificate bundle and use it