Zentyal uses templates (stubs) to load into the redis stored data and to generate the files of configuration for the services it manages. You can read about this in the Zentyal documentation
https://doc.zentyal.org/en/appendix-c.html#stubsYou have to locate the Zentyal template containing the needed parameter:
root@khazad-dum:~# ls /usr/share/zentyal/stubs/openvpn/
default-openvpn.mas openvpn-client.conf.mas quagga
noebox-openvpn-client.conf.mas openvpn.conf.mas systemd.mas
Afterward you have to copy the template in a special folder:
root@khazad-dum:~# mkdir -p /etc/zentyal/stubs/openvpn
root@khazad-dum:~# cp -vfp /usr/share/zentyal/stubs/openvpn/* /etc/zentyal/stubs/openvpn/
You have to work on the template copy that you did into
/etc/zentyal/stubs/openvpn . Zentyal is designed to search the templates in the /etc/zentyal/stubs/ subfolders and, if these doesn't exist it search the template in the original /usr/share/zentyal/stubs subfolders
To make effective your changes, you'll have to restart the daemon after some customization on the templates.
You can add the neeed parameters into the templates that generate the correspondant files of configuration. The only condition is to use openvpn valid parameters and values and to load them in the correct Zentyal template.
Cheers!