No, that is not possible at this moment. The plugin is for shared hosting where each customer can enable SSL for its domains or subdomains. There is one SSL lineage for each domain/subdomain.
Which subject alternative names you would want to add? For which purpose?
We could add support for subject alternative names but a customer would be restricted to names that are part of its domains.
If I understand correctly, you want add each customer domain name as subject alternative name of the panel SSL certificate.
Problem:
Another thing:
nginx doesn't bind to a specific domain, so that's not an issue
That not really true. This works only because currently, there is only one server defined on nginx side (the panel) which acts as a "catchall" (default vhost). In near future, nginx will be also available for customers, meaning more than one vhost and thus, the panel will not always be the default vhost.
To resume: Your use case is something that we cannot really take into consideration. We are working with really big hosting compagnies for which such feature would be totally useless due to limitations listed above. As I understood, you want create a SAN SSL certificate containing customer domain names as subject alternative names but most of time, those domains are not known yet when the SSL certificate for the control panel is issued.
So, here the only thing we can do is adding a textarea for subject alternative names but it will be available at admin level only, and only for the panel and services SSL certificates.
Note also that if one domain doesn't points to your server (for any reasons) there will be a failure during SSL certificate issuance. There is a certbot option (--allow-subset-of-names) to ignore names for which there is a auth failure but we don't enable it by default. And doing a DNS check for all listed subject alternative names, as we currently do for customer domains, panel domain and system hostname would be a crazy thing. 99 subject alternative names would involves at least 99 DNS queries.
So, here the only thing we can do is adding a textarea for subject alternative names but it will be available at admin level only, and only for the panel and services SSL certificates.
that would be exactly the thing I would like, but I see and understand your concern. The limitations wouldn't be an issue for me, since it's quite small and I wouldn't hit either limit, but I get, that you have to look out for the big customers as well.
Thanks for your post, and if you do plan to create such a text field in the future, I would be very happy 
It is clear enought for you?
Note that for the services SSL certificate, the www.XXXX SAN is added by default only if you use the domain as customer domain (or subdomain) and if you included www.
It is clear enought for you?
that's perfect!
Crazy thing 
