Thread tagging was a feature requested since day 1 with huge support from the community: Implemented - Thread Tagging
However it got implemented 5 years later... obviously most people don't have the patience to wait 5 years specially when there is an add-on that adds tagging functionality: [Tinhte] XenTag
Many, many people are already using XenTag. While the add-on works fine, we all know that it is always better to use the core feature if it is available.
So @Chris D @Jake Bunce or @Slavik could you please provide us the SQL query(ies) that will read the existing tags from the database and copy them to XenForo's own tagging system?