I am using i3wm and Libreoffice. I have already installed the libreoffice-kde package. Currently LO seems to be integrating with gnome by default, but I was wondering if there is a way to force it to use KDE integration?
You just need to remove any other integration packages and leave only libreoffice-kde integration. How you do it depends on your distribution (I'm assuming you are running linux).
As an example if you are using ubuntu you need to execute the followings
sudo apt-get install libreoffice-kde sudo apt-get purge libreoffice-gnome libreoffice-gtk libreoffice-gtk3