It is quite simple to add an Addon Domain with the help of cPanel. As you have already an existing domain in your closet for your website, now there is an addition of another domain for your website if you want to add. But firstly it is really important to know about Addon Domain.

What is an Addon Domain?
Addon domain is an additional domain of the website that is store in the system as a subdomain of the main website. It is completely independent of the Primary domain and also it is possible that it may have original content.

Now, it will be discussed as to how to add an Addon Domain with the help of cPanel:
  • Log on to your cPanel
  • Choose the Addon Domains option that is mentioned under the heading of subdomains
  • Enter the Add on Domain name field which you want to keep
  • Enter the name of the directory
  • After Adding an Add on Domain, keep the proper password for your Addon Domain

After doing this process, your add on Domain will be added & you can add content whatever you want. There is also a proper Guide on adding an Addon Domain in cPanel & also how to remove an Addon Domain, you can read this to get knowledge about it & apply for your website.