Answer:
Explanation:
The easiest route is Forwarding the existing conversation to another person. The previous thread will be quoted in the new message.
The method in the question appears to be for preserving the top-down reading structure of the existing thread, of which there is no way I'm aware of. Often times, the aforementioned method of simply forwarding an email is enough to impart some context to the person.
Alternatively, one might view the entire thread, and create a PDF of it (possible from the Print dialog box on a Mac, or to save as a PDF in the File/Edit menus in certain windows applications), which they could then send in an email.
