If you can wait, I’m preparing instructions on how to transfer an existing gen2 refcat (we have a PS1 refcat in the LSST format) to a gen3 repo. It should only be a couple of commands. See DM-30624 for updates.
Converting an external refcat to the LSST format and then ingesting it into a gen3 repo is work I’m doing currently; DM-29543 is the right ticket to follow that.