Yes, you can have an Add-On Domain on your account. According to the hosting plan that you are on, you may be able to have more than one add-on domain on your account.
If you click on the Add-On Domain icon in the control panel, you'll pick the domain name and the username and password. The username is a FOLDER that is created by the system. This folder must not already exist.
Once you create the add-on domain you will be able to upload files into that folder on your site.
The resources of the hosting plan are shared between your main site and the add-on site. There will be only ONE control panel.
Please be aware that Dedicated IP numbers and SSL Certificates can not be installed on an Add-On domain.