I messed with my boss's OpenSuSE 10 installation a few months earlier installing a KDE for him and getting the menu item to appear in the window manager list in KDM/GDM. I don't have access to the installation now so I no longer confirm, but this is recalled from my memory.
The system was originally installed with only Gnome.
Just use the package manager and install with the CD-ROMs/DVD-ROM/whatever, and select KDE. Depending on the interface you can reach, the approach may be different. I reached the individual package selection screen (the disaster!). The only sensible way to proceed was to select some known sub-package of KDE, say kdepim. YaST will then calculate the dependencies upwards and select all other packages needed.
But my experience (probably just unfortunate enough) was that even many KDE packages was installed, the window manager list may not get updated if you don't have one of the package installed which don't get automatically selected, which modifies the window manager list. The package name was kde-session or similar, but I forgot. You need that package before the package list will be modified.
If you don't have anything critical on it you may just do a fresh reinstall, though if you would like to experiment (like I did) you may wish to try it out adding an entire KDE after the Gnome system was installed. That is likely to kill you some time.