SSL/TLS Certificates for Addon Domains
You are able to install an SSL certificate on any domain in your cPanel account.
You are allowed to purchase and install an SSL certificate for any domain. However, you are only allowed to have one SSL certificate per hosting account.
When you purchase an SSL Certificate through your cPanel, it will give you a drop-down menu to choose which domain you want the SSL Certificate installed on. You will just choose the domain you are purchasing the SSL Certificate for, and continue with the normal purchase and installation process.