adding SSL to existing virtual server with active subdomains

I already have SSL with a commercial cert set up for the server’s ‘own’ domain (and copied to webmin, usermin, dovecot and postfix). That’s all working fine. I use external DNS services and so have local DNS services disabled.
IF I was setting up a new virtual server, the set-up process includes being able to specify a Virtual Network Interface and I could use one of the additional IPs assigned to my physical server. However, I need to activate SSL (with a wildcard cert) for an existing virtual server with active subdomains, and in that case things don’t look so straightforward. It’s a production site so I’m reluctant to just try things hoping they’ll work, and so I would appreciate some step-by-step guidance if possible.
Thanks in advance.

There’s an open support request for this here: