You can layer locally by downloading the rpm package you want to install and use rpm-ostree install <pkgfulllocalnameincludingpath.rpm>. This means no auto updating which you would need to take care of. I currently use chezmoi which is layered locally, at update time I have to uninstall my current version then install the updated version to update that pkg. Everything else is auto-update through the OS, even layered stuff from known repos. If you need to add a repo, it is still done by copying the repo file into /etc/yum/repos.d or by using the sudo ostree refs command (see ostree --help for details). When you add the repo, it should auto-update the pkg when rpm-ostree does the check normally.