Yes. In digiKam, face recognition can be run for a single photo, provided the software already has tagged reference photos of the same person to compare against. In practice, you can select the image and use digiKam’s face recognition tools on that item rather than rescanning the entire library. If no face is detected automatically, you can still add the face region and tag manually. Recognition works best after you’ve already tagged a few photos of that person, since digiKam needs examples to learn from. For exact steps, the digiKam user documentation/wiki covers the single-photo face recognition workflow.