Hello there,
On one hand, with regards to Zentyal's OpenVPN implementation, it is correct that the Zentyal GUI doesn't provide the option to configure Zentyal-to-3rd party VPN server with user and password. You should manually configure this through the templates. Did you already try this?
On the other hand, when posting in this Forum, please keep in mind that this is a Zentyal Forum. In the same way that the help requests should be related to Zentyal, the answers should help the users to solve their issues within the limits of Zentyal.
Best regards,