Yes, you can absolutely use Domain+ with the Web App (beta)!
At this time, you can only subscribe to Domain+ when you first sign up for the Web App. This means you can only have one Domain+ site via the Web App at the moment. However, if you have an iOS device, you can use the Universe app to purchase Domain+ for new and existing sites. We are working on making it possible for you to upgrade existing Web App sites to Domain+ soon!
Need more help? Reach out to our Support Team via email at firstname.lastname@example.org.