No, transitions are compiled in. If you are familiar with building your RPMs, install the kdeartwork source rpm, untar the source file, open kdeartwork/kscreensaver/kdesavers/slideshow.cpp, and there you will find the effects listed under kSlideShowSaver::registerEffects().
Edit as desired, recreate the compressed source file, and do an rpm -bb kdeartwork.spec to recreate the binary rpm to reinstall.