The digikam tags are just stored in a database, so if you shut it down first (or are using a non-sqlite database in recent versions) you can add tags in bulk using raw SQL, restart digikam and write the lot out to the images with a couple of mouse clicks. :)
(but, to be honest, a lot of the time tagging depends on the contents of the images, and for that a bunch of select-mouse-clicks and tag clicks are actually a *good* user interface. Using WIMP GUIs for things that actually are graphical in nature seems like a good idea to me.)