if you have an existing post that you want to show in a different channel, you can edit the message and cross-post. This will add the top-level post to the different channel, but will not include the full set of replies. We will keep this request in mind as we evolve the cross-posting feature in the future and update if the status changes.
Details on cross posting can be found here: https://support.office.com/article/cross-post-a-channel-conversation-in-teams-9c1252a3-67ef-498e-a7c1-dd7147b3d295.
Thank you for your continued feedback. As many people have pointed out, this is available on mobile today. Support for replying to a specific message is on the backlog for desktop. I’ll let you know when we have progress to share. Thanks.