While many NativeScript applications are designed from the beginning with NativeScript, it's very possible to add NativeScript to an existing iOS or Android application. This can be a very good thing if you'd like to extend an existing application and move towards a cross-platform architecture. We've put together more detail for each platform:
Android: https://docs.nativescript.org/integration-with-existing-ios-and-android-apps/extend-existing-android-app,
iOS: https://docs.nativescript.org/integration-with-existing-ios-and-android-apps/extend-existing-ios-app
