Tagging isn’t that useful or accurate, its really just a glorified search field.; not to mention its something that the end users don’t really use that often.
If you really want to do it, then its a case of running through X posts a time searching for your own predefined keywords/tags and iff they match then adding the tag to the database. That would be outwith BBpress though, just running an PHP page with SQl on it.
I wouldn’t suggest it though. I recently did it with a football website and found it to be 1) a pain 2) less accurate than i thought and 3) pretty useless.