I found out, how to do it! Here is an example: https://build.opensuse.org/package/s...TO-libXt-devel
Upload the original rpm and create a spec file that installs the rpm into the %{_rpmdir}, then it is added to the repository. Awesome :-)

The package builder seemed to have used a program called "import-rpms 1.0.0" but google did not find any usable results. Anyone knows about this?