KDE4 (at least up to KDE 4.5) unfortunately comes with its own root CA bundle, but without any tools to modify or extend it. The cleanest way is, at the moment, to replace it with the bundle managed by the respective Linux distribution. For Debian or Ubuntu, it is simple enough to prevent future KDE packages to overwrite the bundle again and therefore undo the changes made:
- sudo dpkg-divert --local --rename --add /usr/share/kde4/apps/kssl/ca-bundle.crt
- sudo ln -s /etc/ssl/certs/ca-certificates.crt /usr/share/kde4/apps/kssl/ca-bundle.crt
This will use the CA bundle managed by the ca-certificates Debian/Ubuntu package for all KDE4 applications and will prevent packages from updating the file.