I see this misconception all the time about Fedora Atomic distros. You can actually install any normal package available through the included repos, or add your own repo (rpm-ostree install $pkg). DNF can be used to add a repo from a URL and then you just use rpm-ostree install $pkg . It is really that simple.
The reason you aren’t supposed to is that it makes the system diverge from the default image by overlaying the package. Still though, Fedora Atomic is just Fedora but container images for updates.
I personally adhere to the idea of avoiding installing too many overlayed packages. Most i have installed in like five (with dependencies) at once. If you are comfortable with still using mostly Flatpaks and (only) a few overlayed packages, then Atomic may still be for you.
I really do recommend Secureblue.