I’ve added basic support for threads to my other pending PR (changes the same bits of code):
Either syntax can be used:
/discourse post thread https://<slackname>.slack.com/archives/C6029G78F/p1522952993000017
or
/discourse post https://foo.slack.com/archives/G49KKS6AX/p1523062012000911?thread_ts=1523091738.000088&cid=G49KKS6AC
In both cases the “first/last message” UI is skipped - the entire thread is included in the transcript.