I use Thunderbird's threaded view of email messages to group emails together which are related. However, sometimes people will reply to messages using some mail program which does not properly set the headers to tell it's a reply, or will even write a new mail instead of replying.

In these cases I would like to manually assign or "dock" a mail to an existing thread. Is there some way / addon to do this in Thunderbird?

I'm thinking along the lines of a context menu "attach mail to thread XXX". The mail would then become part of that thread (maybe with a special marker explaining that it was manually grouped).