Yes, all unique domains. All you need to do is point the DNS for the other domain names to the same DNS as the main domain that you ordered hosting with.
DNS information is found in your instruction letter, or you can do a WHOIS lookup on your domain at InstaPro.net To do a WHOIS just type your domain in the search box like you are trying to register it. It will tell you the domain is registered, and to click a link for more details. Click the link and follow the on-screen instructions. Eventually you will receive some information on the screen and your current nameservers will be listed.
Then all domains will resolve to the same server. Once they have resolved, you can use the option in your cPanel called "addon domains" then type in the domain name, the folder you want to act as public_html for that domain, and an account password for that domain.
The password allows you to setup a separate ftp account so that you can only access that folder you specified for public_html for the secondary domain, however it is not needed as you can always log into the main website and load files into the secondary site from your main login.