Force all domain webmail and subdomain creation to another subdomain

As the title suggests, how do I go about changing the "template" cPanel use for webmail access (default webmail.domain.com) for users behind firewall that prohibits access to cPanel services?
I'd want all current and new domains to be redirected to the subdomain "ygm.domain.com" as I've already purchased a SSL for ygm subdomain instead of webmail subdomain.
Also, if new domains were to use webmail.newdomain.com, users will be thrown a SSL error as the SSL cert provided is ygm.domain.com therefore I'd like by default if clients go to theirdomain.com/webmail, instead of being redirected to webmail.theirdomain.com, they will be redirected to ygm.domain.com.