One thing to consider before you proceed with converting a stand-alone cPanel account into an addon domain under another cPanel account is that Acenet suggests keeping accounts separate for securirty purposes and other reasons. You can view more details on this in our article [Unlimited domains with Shared Hosting vs. Reseller Hosting]
To move a website from a stand-alone cPanel account to an addon domain of another cPanel account, there are three things that must be moved:
- Website files
There is currently not an automated process to do this, but cPanel is working on it.
The steps to do this are:
 Create the Addon Domain on the new cPanel account using a dummy name (i.e. example.com.zz). If you're not sure how to create an addon domain, you can view our knowledgebase article on how to do so here:
 Create any subdomains the addon domain may need. You can view our knowledgebase article on how to do this here:
 Copy all site files from the old account's public_html to the addon domain's document root folder (which is specified during the addon domain's creation) in the new account.
 On the old account, backup all databases and move the backup files to the new account. Name them similarly if at all possible to cut down on confusion.
 Create blank databases on the new account, and then import the database backups from the old account into the appropriate new databases.
 Edit your script(s) to reflect the new file locations, database(s), usernames, and passwords. The majority of the time this only needs to be done in the script's configuration file(s) but you will want to consult with your webmaster to ensure all of the appropriate files are edited.
Now, mail is not quite so straight forward. You will have to create all of the email accounts on the new account and have your users log into the old email account and the new email account simultaneously with IMAP using their preferred email client (Thunderbird, Outlook, etc.). This will make it so that you can drag-and-drop the mail messages between the old and new accounts and copy the mail.
You can view our knowledgebase articles on how to configure some popular email clients here:
Once you are ready to switch over, change the domain name on the original account to a new dummy name (i.e. example.com to example.com.old) and then change the addon domain to the original domain name (example.com.zz to example.com).
If the site was moved to a different server, you will need to point the domain(s) to the new server's name servers at the domain(s) respective registrar(s). It will take 24 to 48 hours for the domain(s) to fully propagate to their new name servers once they've been updated.