Interoperability in healthcare information systems is crucial for seamless data exchange. Legacy systems often employ the Entity-Attribute-Value (EAV) model, which poses significant challenges for data standardization and integration with modern frameworks like HL7 Fast Healthcare Interoperability Resources (FHIR). This paper presents an easily reusable open-source implementation of a FHIR façade for EAV-based legacy health information systems based on HAPI FHIR. The proposed methodology separates the mapping logic from the implementation code, allowing for easier maintenance and updates. We demonstrate how this approach addresses key challenges in transforming legacy EAV data to FHIR resources, particularly focusing on the management of multiple source forms and the need for a maintainable solution. Initial results show that the approach is effective, achieving high accuracy in data transformation while maintaining system stability and ease of maintenance.