This idea has been developed and deployed to Canvas
In the iOS mobile app, the message center allows you to reply to any and every message received, but it does not let you add or remove users from the original recipient list. Creating side conversations or adding key people to the message are simply not supported.
Please consider a feature that would allow To: users to edit the recipient list (but not those on the CC: list). This would improve the flexibility of the iOS app (and if it is true for Android, then maybe there too).