The "webserver" functionality of create-react-app is based upon webpack (wepback-dev-server) and not on basic node/express. To change that and to combine it with node/express api inherently, this would probably require a major rewrite in "manual mode" of the create-react-scripts.
An easier solution is to run both in parallel and for the purpose that the client can work on the same port 3000 (to avoid cross-origin), webpack-dev-server can be configured as a proxy for node/express (running on 3001).
A very good tutorial for that could be found at: