Add-ons (now apps) can't be done without HAOS. It's one of those non-obvious things.

There's other things I forget which are also more difficult/annoying to manage on the container version.

I have a rather large docker compose stack so my first experiments with HA were as running it there.

That lasted maybe a week before I went and ran it as a VM instead.

Apps/add-ons is just another container, so you can add them manually in the compose file.