It should work, if you have a plain React-app, like the one create-react-app creates. In this case, you simply take a page-component and put it into a <Route/> within your <SinglePageApp/>

It could be a problem, if you used functions (like theming and content management) of Stackbit, Gatsby and Netlify. In this case, you have a whole software stack. This would be problematic to migrate.