Comment 23 for bug 1770003

you could preseed your installer to install the kernel in the PPA. I have not tested the instructions below.. but should give you an idea how to preseed. From your comment above looks like you might not have a booting system to be able to install the new kernel etc.

Assuming you are using a netboot install.

-- tell installer grub to use preseed --
Edit ubuntu-installer/ppc64el/grub/grub.cfg and add your preseed file. Like for example:

menuentry 'Install' {
    set background_color=black
    linux /ubuntu-installer/ppc64el/linux auto=true priority=critical url=http://<web server>/preseed/preseed.ppc64el --- quiet
    initrd /ubuntu-installer/ppc64el/initrd.gz
}

-- preseed example --
You can use late-commands in your preseed file to install the PPA kernel like:

# Install kernel from PPA
d-i preseed/late_command string in-target add-apt-repository -y ppa:ubuntu-power-triage/lp1770003
d-i preseed/late_command string in-target apt update
d-i preseed/late_command string in-target apt install -y linux-image-unsigned-4.15.0-20-generic linux-modules-extra-4.15.0-20-generic