Turning an existing app into a PWA
A common question that we hear is "Is it possible to turn an existing web app into a Progressive Web App?". The answer is yes, but with caveats.
Since PWA are just web applications with a manifest file and ServiceWorker, it is definitely possible to enhance an existing application with PWA features. The problem with retrofitting older applications is that they are often not designed to be responsive and their communication is not designed to handle intermittent connections. That said, as PWA are progressive enhancements, you can work on enhancing your existing app step by step. It may not become a "true PWA" overnight, but every step you take will enhance the experience for your users.
Microsoft has created a tool that helps you generate an App Manifest and ServiceWorker for an existing app that you can use as a starting point. You can try it out at pwabuilder.com.