Raspberry Pi, possible to add packages?

[bluebob]

I'm building a mixed kairos cluster with amd64 and RPi4 nodes, but have run into a confusion. I want to be able to generate a new persistent partition and add the Ceph OSD package to the base image (so my RPi4s are dual-hatted as k3s agents and Ceph OSDs)
Problem is that while this looks relatively straightforward on amd64 kairos, the 'add app to image' part on RPi4 has got me scratching my head. Anyone want to point me at the bit in the doc that I've missed or aren't reading correctly?

I'm wondering if perhaps I need to do a full build-from-the-ground-up as is outlined here: #1903