The newest version of the "Send Message" block in the Slack connector allows you to send a message to another message's thread.
This is done by specifying the root message's ts number in the "Thread Ts" input field on the Send Message block.
Either store a message's ts number when it's returned in the Send Message block's response when a message is created or retrieve it by using the "List New Messages Incrementally" block.
It's also possible to broadcast the thread message to the channel by selecting "true" for the "reply_broadcast" parameter.