add recipients to existing private messages

Say, for example, i'm having a discussion with Seidon and, after having exchanged a couple of messages, we'd want to share this message discussion with somebody else. Is there a way to do that?

There doesn't seem to be a checkbox or a button to that effect - or at least i'm not able to find it.

Thank you in advance.

 

Could you quote everything and then start a new private message with multiple recipients?