The current default directory for kdm sessions desktop files is /usr/share/apps/kdm/sessions. There were 45 files there on my sid system with KDE 3.2.2. It appears kdm does a great job of sorting out which of these is actually installed, then throws the right ones up for me to choose on the kdm session menu. No editing of /etc/kde3/kdm/kdmrc is required to install a new session manager. No SessionTypes list need be entered. The KDE 3.2.2 session editor is defunct now anyway. Just put the appropriate .desktop file in the de facto default directory, and we're in business. I am not sure how well the other desktop managers cope with .desktop files which point to nothing. Maybe that is why they do not want to impose all these files onto another directory shared with other desktop managers.
Point is, until the desktop managers learn to play together or include needed .desktop files in their sessions list directories, for the sake of all of the users up to their knees in blood, I humbly request windowmanager/desktopenvironment/session package maintainers put their .desktop files in the actual current working default directories of each desktop manager. ie for kdm the directory is /usr/share/apps/kdm/sessions/ and for xcfe4 the file is xfce4.desktop.