Hi Jason,
Whenever you add any addon domain from your main domain's cPanel interface, a default FTP user name will get auto created with the addon domain's folder access. With this FTP user name, you can directly access your addon domain's directory.
If in case, you want to add any additional FTP users for the addon domains then, you'll just need to create them with the main domain name but, make sure that you' are typing/entering the addon domain's folder name in the Directory:/ option like as follows.
So ultimately, all the FTP accounts will contain your main domain name in their names, but they will have different directory access levels.
Do let us know if you've any more questions..
Regards,
Victor,
Support Team.