This undocumented feature of cPanel allows you to FTP via an Addon domain username if you replace @yourmaindomain.com with any active domain listed on your account.
You could create a username like
test1@yourdomain.com and when you login to your FTP client you replace yourdomain.com with addondomain.com, like this
test1@addondomain.com. The password and other settings do not change.
The main FTP account can be used to access all the domains, files and folders on your account. Simply log into your FTP client and navigate to ~/public_html/addon_domains_folder/ and upload your files there.