The ability to do this already exists. Click on a message that has tags, select all the tags in the tagger by clicking past the last tag and dragging to the beginning, press Ctrl + V to copy them, select one or more emails, click in the tagger and press Ctrl + C to paste the tags into the tagger, press Enter to assign the tags.

Good suggestion.

I do want to add that if you ‘right mouse click’ in the ‘Tagger’ area then a menu option appears to tag the entire conversation, but it would be helpful if this was automatic.

Also, what a lot of people do is let some tags ‘travel’ on the message too, as they way any replies come back on the thread tagged, although I understand that isn’t how everyone is comfortable using tags.