Here's the bottom line about add-ons --
It's simply not possible -- Home Assistant OS is an appliance type operating system providing the Supervisor. The Supervisor and all add-ons are Docker containers. To my knowledge there is no working version of Docker available for FreeBSD. Without Docker, there is no chance add-ons.
There no getting around it,
add-ons are convenient but you don't really need them. Add-on are just Docker containers that have been customized to allow a more "out of the box" working setup for Home Assistant. They also provide ingress trough the Supervisor, which allows them to be configured from the Home Assistant UI.
If you don't have add-ons, you can just install and configure the software as you normally would. On TrueNAS CORE, you might start by looking at other TrueNAS Plugins or manually create a jail and install what you need. When I am unable to find the software I need on FreeBSD, for example ozwdaemon, I run it from
a Raspberry PI I have setup with Docker.
If you want to run Home Assistant with add-ons from FreeNAS or TrueNAS CORE, you'll need to run Home Assistant OS in VM.
Here's a guide on Home Assistant forum to help with that.
EDIT I just happened on this, so I thought I 'd add it here
Home Assistant OS
* Image based install, add-ons, no ability to “tinker” with the OS on the host
Home AssistantContainer
* Docker on Linux, no add-ons, do what you want on the host - just learn enough Docker to know what you’re doing
Home AssistantCore
* pip install, no add-ons, do what you want - just learn enough Linux to know what you’re doing
Home AssistantSupervised
* Scripted install version of HA OS on your own OS, add-ons, only officially supported on Debian Buster - “just” learn enough Linux and Docker to ensure you stay supported, and remember that OS upgrades may cause the Supervisor to refuse to let you upgrade anything until you fix the thing it complains about