Yes, there is a quite simple way to do this!

First, place your expand-script in a good folder, like `/usr/local/bin/expand.sh`, and make it executable with `chmod +x /usr/local/bin/expand.sh`

Then, edit `/boot/config.txt` and add the following furthest down:

    [expansion]
    init=/usr/local/bin/expand.sh


Reboot, and then observe the expanded filesystem with `df -h`