For anyone whose interested you can use a mixture of Rich Trouton's First-Boot-Package-Install and Greg Neagle's bootstrappr to install the quickadd.
The trick is to create a package (I used composer) and add it along with Rich's First Boot Package Install to bootstrappr. The package has a LaunchDaemon and a script, the LaunchDaemon triggers the script which removes the LaunchDaemon and reboots the machine at this stage First Boot Package kicks in and installs the quickadd (First Boot Package needs that restart to install the packages). Note that in 10.13.4 onwards the install log no longer appears for First Boot Package, the process still works you just need to wait until the machine reboots (usually around 5 minutes but I am only installing the quickadd).
Here is the LaunchDaemon
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>uk.edu.hogwarts.fbp</string>
<key>ProgramArguments</key>
<array>
<string>/Library/Scripts/fbip_install.sh</string>
</array>
<key>RunAtLoad</key>
<true/>
</dict>
</plist>
And here is the script
!/bin/sh
Created by John A. McClements on 26/02/2018.
#Remove LaunchDaemon
rm -r /Library/LaunchDaemons/uk.edu.hogwarts.fbp.plist
reboot
exit 0
You can either automate the image from there or if like our environment you have multiple images you can log in with your local admin account and scope the various images to be available in Self Service.
