Hello, on my system (Linux + Digikam 7.9 and 7.10) I create a tag on the right side of the app (tab Tags), then I select a serie of raws+jpgs and say “Apply”. The tag appears now on all the selected photos, including the raws.
Strange enough I can’t find an easy way to copy captions and titles from one photo to another. According to this thread on pixls.us this isn’t implemented yet in Digikam, but in the same post others describe a workaround that does work indeed.
Alternatively, use exiftool to write titles, tags and captions to a series of photos. Type ‘man exifpage’ and scroll to the bottom to ‘Writing examples’. Other tools may work as well (exiv2, jhead, imagemagick…).