Dear Pixelsmack, this has been possible for a long time, at least in Trillian 2.x. Simply click a few contacts holding the control key, then right-click and choose "Mass Message", or create a Meta-Contact, which you can subsequently use to spam...
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finalcut, just choose Tools -> Import Messages -> From UNIX-mailboxes in The Bat, and select your Thunderbird mailboxes. It isn't possible to automatically import all the folder structure, alas.
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