Alright, I want to be clear when I say if you use these applications, you use them at your own risk. I am not responsible for anything you do.
The
link is two applications I threw together to import my MySQL messages into YAF.
I exported my MySQL posts to an Excel file.
Then used YAF-AddMessages takes imports the messages from an Excel file.
Then use YAF-Maintenance to sync up forum categories, topics, and whatever else you find in there.
Hope you can modify these to help you.