It is possible to add the KDE desktop to LMDE. It will not be perfect as some of the Gnome characteristics may show up on the KDE desktop. I have never tried this with KDE as I am not a fan, but installing LXDE on LMDE-Gnome works fairly well without much crosstalk between the desktops.
The KDE meta package, KDE-full is available in Synaptic Package Manager, but it is not KDE 4.6. Apparently it will install KDE 4.3 or 4.4. There are packages for both available and KDE-full does not identify which version it installs. I saw nothing for KDE 4.6 in Synaptic for LMDE. It may not be in the Debian repos yet.
Found this site:http://pkg-kde.alioth.debian.org/
It is the KDE-Debian maintainers' website and it lists the latest version of KDE available for Debian as 4.5.
If you really want KDE 4.6.2, you will have to download and install it yourself as it is not available in Synaptic.