According to this documentation page, currently it is not possible to convert Webex recordings to MP4 format on Mac. I imagine this can't be that difficult to support. Webex should be able to execute the conversion on their server, and then I should be able to download the MP4 file directly.
We conduct training events for dozens of people, and the Network Recording Player doesn't always work for them. It also means we can't upload our own recordings to other sites or servers for people to reference.