From the STORM dashboard click on Websites

On this page you will see a list of Websites that you currently have set up on your Nimbus server. Click the cog icon next to your website to manage your website.

Click on Domains

Click Add Domain

Type in the desired domain name, to use the domain as an alias leave the Redirect box unchecked and click Create Domain

DNS for the alias domain will also need to be pointing at your STORM server, see this guide