That is just how cPanel works.. you need to give it a subdomain name. Therefore, you need to be have access to create subdomains. You should have an option to setup "redirection" where you can specify a folder or file to redirect to. You would still FTP in using the domain owner's username and just place files in the directory you setup for the redirection.
An add-on domain is merely a domain alias that gets its own virtual host entry in httpd.conf (which is setup as a subdomain with the add-on domain listed as a ServerAlias for some strange reason).