cPanel has yet to offer this option although it can be done.
In order to park a domain on top of an addon domain, please do the following:
1 Login to cPanel and click Addon Domains.
2 In the field to the right of "New Domain Name", type the domain name you wish to park on top of your addon domain.
3 Click outside the text box and your other settings will populate.
4 Under "Document Root:", erase everything except "public_html".
5 Look down to your current addon domain and its "Document Root".
6 Take the "Document Root" and insert it in the field to the right of "Document Root:" at the top.
7 Insert any password in the password field. (You won't need to remember this password, so just use the Generate Password button).
8 Click Add Domain!
You have now just parked a domain on top of an addon domain.
Note: If you receive an error similar to this:
Error from domain wrapper: Domain already exists, it was not added.: example.domain.com
This could be due to one of two reasons:
1 The domain you are adding has already been added and therefore already exists. So, you will need to delete the domain from Addons, Parked or Subdomains before proceeding.
2 The subdomain which cPanel automatically generates already exists. When this happens, you can manually change the subdomain which cPanel generates for your addon domain.
EXAMPLE: If you are trying to park example.com on example.com.au, cPanel will fill in the "subdomain/ftp username" field with "example" (which may already exist). Just change the "subdomain/ftp username" to a new variation, like "exampleau".
