yes, you can add to a kde4 installation e17, gnome, lxde, xfce and other desktops
on the kdm screen, where you select user and password you can press the session type button and select on that list the desktop you want load
when you add it to your system you don't boot to it, it is just a option, a optional desktop environment, it will have access to all your kde4 apps, your same home folder and all your resources already configured in kde4
kde4 will have access to the new e17 apps too