Update: Managed Export and Import is available.
Right now there is no import/export service for Cloud Firestore. We are working on this, it's at the top of the list of things we need to do before we can drop the "Beta" label.
You're right that Firestore is powered by the same technology as Datastore, but the two database are not exactly the same which is why you can't use plain-old Datastore export.
For now the best thing to do is to write your own import/export scripts using either the Java, Go, Node.js, or Python SDKs.
