In principle XSLT can be used to convert between HL7 V2 Messaging, CDA, and FHIR formats. I have actually done this to an extent. But it's a huge mess and difficult to maintain or even understand. Most implementers have now moved on to other technologies for healthcare data mapping and transformation. And format conversions are only one minor piece of the interoperability puzzle.
The standards bodies in the clinical interoperability space aren't accountable to the FDA (although the FDA is a registered organizational member of HL7 and contributes to standards development as a peer to other members). Services like Metriport aren't FDA regulated medical devices. The standards bodies don't inspect implementers.
There are open source libraries for dealing with the data formats used for interactions between providers and health plans (insurers), primarily ASC X12N and NCPDP. The standards documents themselves are somewhat expensive. But Metriport doesn't appear to be playing in that space so it's a moot issue.