Yes, you can create subdomains of your username.opalstacked.com domain, like app1.username.opalstacked.com and so on.
You also have the option of attaching multiple apps to a site on different URI paths, so you can test them all from the same domain. To do that, go to https://my.opalstack.com/routes/ and click the edit icon in the right-hand column for whichever site, then add the additional apps to the site there and save the site.