Yes. You can install as many DEs or WMs as you want without reinstalling.
To install KDE, open a terminal and type in "sudo apt-get install plasma-desktop"
After it's installed, logout and select KDE as your session and log in.
And BubbaBlues, I don't know where you got that idea from. I have KDE, LXDE, and Xfce on Debian Wheezy running, no problems ever.
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