If you have room on your hard drive, I would recommend a fresh install of Mint KDE in dual boot with Mint Main Edition rather than installing the KDE desktop on your gnome install. I think the full install gives a better feel for the KDE experience than a desktop transplant.
Having said that, if you prefer to just install the KDE desktop on Gnome, open Package Manager and search kde-full. Install this package and you will be able to choose whether to use KDE or Gnome at the log-in screen - click "Sessions" and choose which one you want.
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