If you already have a hosting account for the domain name then all you need to do is to change the DNS settings for your domain name and use the NS's provided to you by the web hosting provider. If you want to add it as an addon domain then you will need to follow these steps:
1. Login to your cPanel account.
2. Go to Addon Domains.
3. Add the new domain in the "New Domain Name" Field.
4. The next two fields will be automatically filled.
5. Fill the last 2 fields "Password" "Password (Again)" and click the "Addon Domain" button.