From EZP Knowledge Base

In order to park a domain on top of an <a href="Addon Domains, Parked Domains , Sub-Domains explained">Addon domain</a>, please do the following:

• Login to cPanel!
• Click addon domains

• Type the domain name you wish to park on top of the addon domain in the field “New Domain Name:”
• Under "Document Root:", clear the auto populated result.
• Look down to your current addon domains and copy the "Document Root" from the domain you wish to park to.
• Take the "Document Root" and insert it in the field "Document Root:" at the top.
• Insert any password in the password field. (You won't need to remember this password, so just use the Generate Password button.)
• Click Add Domain!

Here's an example of what the above should look like: