Auto tagging the Q&A?

I am wondering if it is possible to autotag the questions/answers because it seems unreliable to allow users to tag them, plus I just imported 6000+ questions and some of them have no tags. Is there a plugin, tool, or anything that could make this happen? Thank you in advance.

(BTW I already tried other auto-tag plugins but since QA plugin uses a custom post-type and custom-tag, those plugins don't work...)

Henry