How to install trenchbroom-kp-addon.zip under linux
Install TrenchBroom-Linux-v2019.3-Release.x86_64 normaly or with a software package.

Then in a terminale use the commando:
mount |grep snap
to find:
/var/lib/snapd/snaps/trenchbroom_75.snap on /snap/trenchbroom/75 type squashfs (ro,nodev,relatime,x-gdu.hide)

This file is only read only To modify files in squashfs, you need to unsquash it, modify the file and then re-make the squashfs file.
sudo umount /snap/trenchbroom/75
sudo unsquashfs /var/lib/snapd/snaps/trenchbroom_75.snap

Then unzip trenchbroom-kp-addon.zip into /squashfs-root/usr/share/TrenchBroom/
So there is a folder /squashfs-root/usr/share/TrenchBroom/games/Kingpin/

To repack it:
sudo mksquashfs squashfs-root trenchbroom_75.snap

Copy it back to its original location and re-mount it.
sudo cp trenchbroom_75.snap /var/lib/snapd/snaps/trenchbroom_75.snap
sudo mount -t squashfs -o ro,nodev,relatime,x-gdu.hide /var/lib/snapd/snaps/trenchbroom_75.snap /snap/trenchbroom/75

Now you can use Trenchbroom with Kingpin under linux.
FREDZ.
