I've occasionally used KPhotoAlbum for a few years and eventually added many tags to the pictures.
But I've decided I wanted to try other tools, and digiKam seems to be the best option from the many reviews I've read.
Still, there's apparently no automatic feature to import into digiKam the tags set in KPhotoAlbum.
Fortunately, some smart people have implemented Perl tools allowing to overcome this issue.
The process involves modifying the pictures to save the tags inside the files, using the IPTC standard. Then, digiKam will be able to load the tags from the modified files.
dh-make-perl --build --cpan Image::Kimdabaand
dh-make-perl --build --cpan Image::IPTCInfo
Thanks to Pierre Doucet and Bruno Adele for sharing this. Hope this helps.