Mail - can I manually organize (thread) emails?

I love the threading feature in Mail (View>Organize by Conversation), but sometimes it doesn't work. Often this is because the subject gets changed when somene replies from a device. For example if the subject changes from "How ya doin?" to "[RE: How ya doin?]".

On other occasions, Mail just fails to recognize that two emails are part of the same thread despite the subject line staying the same. It's rare, but it occasionally happens to me (like just now).

Does anyone know of any way to manually thread messages? Or basically force Mail to merge two threads / emails into one thread?

Thanks!