Convert a cPanel account to an addon domain

There is no tool for this. The manual procedure, the order that matters, and the parts that are irreversible.

Move a domain from its own cPanel account into an existing account as an addon domain. Applies to reseller accounts.

There is no native tool for this, and it is outside the support remit — cPanel provides no conversion path, so every step is manual and one of them destroys the source account.

Read the whole procedure before starting. If you are not confident, a developer should do it.

Why it is awkward

A cPanel account and an addon domain are structurally different. The account owns its own home directory, databases, email accounts and DNS zone; an addon domain lives inside another account's structure. Nothing maps across automatically — you are rebuilding it in the destination and deleting the original.

The destructive part is that the source account must be terminated before the domain can be added elsewhere. cPanel will not let the same domain exist in two accounts.

Before you start

Take everything you cannot recreate:

A full cPanel backup of the source account, downloaded and verified. Open the archive and confirm it contains homedir and a mysql folder — see Download a backup of your account.
Email, configured locally. Add every mailbox to a desktop client — Thunderbird, Outlook, Apple Mail — and let it download completely. Mailbox contents are the thing most often lost here, because they are not obviously missing until someone looks for an old message.
A note of the DNS records, if any are non-default — see Reset a DNS zone to its defaults.

Verify the backup before continuing. After the next step there is no source to go back to.

The procedure

Terminate the source account — WHM → Account Functions → Terminate Accounts.
Create the addon domain in the destination account.
Restore the files into the addon domain's document root from your backup.
Create the databases and users, then import the .sql dumps — see Import a large database.
Update the application's configuration with the new database credentials. wp-config.php for WordPress; the names now carry the destination account's prefix.
Recreate the email accounts under the addon domain.
Upload the mail from your local client back into the new mailboxes.
Check DNS, and re-add any custom records.

Expect downtime

The domain is unreachable between termination and restoration — realistically an hour or more, longer for a large account. Do it at a quiet time and tell the client beforehand.

An easier alternative

If the aim is consolidating billing rather than merging accounts, consider leaving it as a separate cPanel account. Separate accounts have real advantages: independent resource limits, independent backups, and one site's problem does not affect the other — see How many sites can one account host.