select the files you wish to work with, then, in the tag window, make sure the [Categories] field is visible. If it is not there, right click in the tag window and pick it from the "Also show" list.
Once there, click inside the Categories field to edit, and type in there: =[comment] and press enter. (don't miss the equals sign, it's critical to your success)
This should copy all your comments over to categories.