Gmail forwarding and thread merging

No, there isn't. You can mute a thread, but you can't cleanly merge them.

One thing you can do: forward an orphaned email to yourself as a response to the thread you want it to be a part of, then delete the original. That won't force your sender to keep things together, nor is it particularly automated, but for reading/archiving purposes it may fit your needs.

The following conditions must be met:

1. The subject must be similar (e.g. test and re: test will work; but test and test 123 won't).
2. The sender must be a part of the thread OR the in-reply-to header must be used.

The in-reply-to header can be used via Gmail's interface by simply replying to the thread. This is what enables forwarded messages to be a part of the same thread even though the sender is different.

If you want more information, you can check out my blog post where I posted more detailed information about my findings.