In a multi-user environment the option should be available when
KDE4 is installed. All we need are some desktop files in
tdebase/tdm/kfrontend/sessions. We need only copy the files from
KDE4:

/usr/share/apps/kdm/sessions/kde-plasma-safe.desktop
/usr/share/apps/kdm/sessions/kde-plasma.desktop

I tested adding the files on a system with KDE4 installed and
another without. With the former the options appeared as expected
and with the latter the options did not appear as expected.

Comments?

Darrell