Yes it is possible. Your first have to create a sub-server, and give it a subdomain name. To proceed, log on virtualmin, and you'll see a menu entry on the left called create virtual server: click on it and fill the different fields.
If you are using you own DNS, you also will have to add the domain name in it. Otherwise, the cbpolis DNS will do the work for you.
The hosting plan you have is limited to 5 domains and subdomains as well as 5 databases. If you need more, you will have to upgrade it. It can be easyly done through the admin portal.