"Manual threading" in Thunderbird?

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).

Update: I'm using Thunderbird 3.7 Alpha, but I'm interested in solutions for any version.

The only solution I can think of is to change the subject-line so that the email would fit into Thunderbird's thread-model (Re: subject). There are two way I can think of:

1. Forward the email to yourself with a new subject-line.

2. Use the TB Header Tools Extension to edit the subject-line. However, I have no experience with this extension and cannot guarantee that it works. Apparently, it used to work pretty well with TB2.

[EDIT] The TB Header Tools Extension is not available for current versions of Thunderbird.
I now recommend EditEmailSubject which I use myself with no problems.

Please check a good info on threading here: https://github.com/protz/GMail-Conversation-View/wiki/What-is-threading

Main reason for unwanted behaviour?:

Unrelated threads are grouped together!

Someone was lazy, and instead of composing a new message, just hit "Reply All", and changed the subject. You need to educate these people and tell them not to use reply all when they intend to start a new thread.

I'm still wondering whether there is a way to manually change the reply-to or references headers.

You can use the Header Tools Lite plugin by genius Paolo.

- select message under which to Thread
- message_context_menu>HeaderToolsLite>Change Header Details
- copy Message-ID: (or one of References: to thread randomly)
- select misplaced message
- message_context_menu>HeaderToolsLite>Change Header Details
- paste into References: in comma separated list
- thank

harrymc's answer pointed me to a review post for the TB Header Tools Extension, which says:

However, I have discovered that TB3 now DOES allow the email to be dragged onto the desktop, edited with Notepad, and dragged back, so that's my preferred way now.

This is probably what I'll do as well. That way I can change/add headers as I see fit (either adjust the subject, or directly add the missing In-Reply-To: or References: header.