Two related questions on this topic:
Can a moderator of a conference call start or stop recording after the call begins? Can they do this multiple times, and i
Twilio evangelist here.
There is no way to start/stop a recording of a <Conference> call through the Record attribute. You might be able to do this by dialing in Twilio as a participant.
There is also no way to modify an existing recording. You would have to download the WAV/MP3 and stitch them together yourself.
Hope that helps.