User-agent:
Mozilla/5.0 (X11; Linux i686; rv:13.0) Gecko/20120615 Thunderbird/13.0.1
Currently this is not possible, but I would also like to see such a
feature implemented. So if anyone is up for the task, I would be glad to
help (the currently unused rednotebook/gui/imports.py would be the right
place to start probably).