In the first version of Federated Properties, enabling the feature in a Wikibase that already contains local properties is not supported. Additionally, when the feature is enabled, there is no option to create additional local properties. This was an intentional decision to limit the scope of our initial MVP.
While Federated Properties is useful in certain contexts (e.g. using a Wikibase for Wikidata staging, or quickly evaluating the software/Wikidata ontology), the above limitations must be addressed before wider adoption of the feature is possible. Feedback from testers of the MVP have indicated that this functionality is a key missing piece.