This is working today in the web client on Edge & Chrome. The team is actively investigating feasibility in the desktop client.
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.
We are working on a design with more density in mind. Until this design is solidified we are putting this item back to Under Review.