Drop in repos into /etc/yum.repos.d/. You can use the same ones RHCOS uses for composes: https://gitlab.cee.redhat.com/coreos/redhat-coreos/blob/master/rhel8.repo
Or alternatively, if the pkgs don't have any deps, you can just rpm-ostree install <URL>
e.g. rpm-ostree install http://download.eng.bos.redhat.com/brewroot/vol/rhel-8/packages/authselect/1.1/2.el8/x86_64/authselect{,-libs}-1.1-2.el8.x86_64.rpm