Import tag hierarchy into darktable from DigiKam ?

If you can make digiKam export its tag hierarchy as a Lightroom-like keyword file, yes. I don’t know if digiKam can, though.

But if you have the tags written in the JPG, then importing them in darktable should work. However, to avoid duplicates it might be better in digiKam to save only the lr-hierarchical:subject field to the JPG, and remove all the others (Hierarchical tag chaos)

Edit: better yet, save all fields in digiKam and remove the unnecessary ones from the JPG metadata using something like exiftool. That way you won’t mess up with digiKam’s export defaults for the future