There is a great number of articles about how to make syslog_ng consume logs from systemd-journald, but not too many that go the other way.

I needed to grab logs off a networked device and wanted to collect them in my journal.

source s_net {
        syslog(
                ip("10.128.1.49")
                transport("udp")
                port(1514)
                flags(expect-hostname assume-utf8)
        );
};

destination d_journal { unix-dgram("/dev/log"); };

log { source(s_net); destination(d_journal); };