Based on what I’ve read, it looks like Ionic is not intended to take existing React web apps and re-target them to be a desktop (and offline) application.
To say another way, it appears that an app must be developed using Ionic UI components in order support multiple platform targets. Thus we would have to re-write our apps to take advantage of the Ionic framework.
If that’s the case, you could just add ionic into a react project and mix them.
Since ionic is just using standard react components, there’s nothing saying you couldn’t mix UI libs. This would at least get you part way with things.