It depends what you mean 'on top of'. If you mean adding some RN to an existing app, that is conceptually easy but practically quite messy (https://facebook.github.io/react-native/docs/integration-wit...).

If you mean adding native modules into an essentially RN app, that's practically very simple (https://facebook.github.io/react-native/docs/native-modules-...) but still requires some conceptual care (like any API design).