SSL for mail sub-domain

We have a FastSSL certificate already installed for the main domain and www, and now I want to add SSL to the mail sub-domain (we are NOT using LetsEncrypt or AutoSSL). I installed a new FastSSL certificate on the server and then added it via 'Manage Installed SSL Websites' but it overwrote the certificate for the main domain. cPanel seems to group the mail, cpanel, webdisk, etc subdomains in the same section, so I can't see any way to install a separate SSL certificate for the mail sub-domain, is it possible? The account is on a GoDaddy server.