repo-add /path/to/repo.db.tar.gz /path/to/package-1.0-1-x86_64.pkg.tar.zst
A package database is a tar file, optionally compressed. Valid extensions are .db or .files followed by an archive extension of .tar, .tar.gz, .tar.bz2, .tar.xz, .tar.zst, or .tar.Z.
repo-add /path/to/repo.db.tar.gz /path/to/*.pkg.tar.zst
/etc/pacman.conf
[custom]
SigLevel = PackageRequired
Server = file:///mnt/repo/Packages
repo-add ./custom.db.tar.gz ./*