For a short time until Thunderbird V5 was released there was a message filter import export add-on.
This appears to have been abandoned by it's original Author, and around the time of Thunderbird Version 19 someone fiddled the max version and again added it to Thunderbird addons at AMO. I have done the same to increase the max version to 38. So the add-on should install for a while yet. I have not used it as yet.but I appear to lack an import option.Looks like I will have to investigate further.
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