There is this PPA:
https://launchpad.net/~kubuntu-ppa/+archive/ubuntu/backports
It contains a package named "plasma-desktop". You must however trust the
people who administrate it. In particular, they could include proprietary
software (now or along a future update).
Some Ubuntu users reported that the PPA has stopped providing packages for
Ubuntu 14.04 (hence for Trisquel 7) since last June:
https://askubuntu.com/questions/668384/how-to-upgrade-to-kde-plasma-5-on-ubuntu-14-04-3
But those packages are listed. So, either I do not understand or the
situation has changed. If you want to try the adventure (despite the probable
problems), execute this command (do not forget the option "-a"!) in a
terminal:
$ echo 'deb http://ppa.launchpad.net/kubuntu-ppa/backports/ubuntu trusty
main' | sudo tee -a /etc/apt/sources.list
You can then open the "Synaptic package manager" (from the "System
settings"), "reload", and install "plasma-desktop".
