I couldn't get ZFS installed via the OMV GUI. Instead I installed it directly via the terminal:
apt install dpkg-dev linux-headers-$(uname -r) linux-image-amd64
apt install zfs-dkms zfsutils-linux
Now, in the GUI go to
System > Plugins and install
Storage > zfs > Pools and add a new pool. I recommend setting the compression to
You now have a working storage pool set up.