Sadly it can only do that with phpBB currently (because its database structure is much cleaner, and the builtin forum is a bit more cluttered).
It wouldn't be TOO hard to figure out the necessary SQL statements to mirrior it internally, though.
Moving comments of an existing topic is not supported by the plugin (and quite hard to achieve, too!); you would need to move existing comments manually to the forum...In particular, I have one post at my blog that has grown to have >50 comments and keep getting more, and would like to take the whole comment thread into a forum post. Would that be possible?
Regards,
Garvin