You don't need to import your library in to darktable. Darktable isn't great at photo management and workflow.
What I do is use digiKam to manage my old Lightroom collection. In lightroom, make sure you've set it up to store metadata in sidecar files. Write your sidecar files out (this can take a long time). Open digiKam, and you can import all of your lightroom photos and data.
Then photos that need editing get sent to darktable from digiKam