If you install the WP Discourse plugin you can publish old WordPress posts to Discourse.
I don’t think so. It’s something that I’ve only tried in my development environment. It would take a few hours of work to get it into a state where I could share it. I’m not sure if it would be a good solution for what you’re trying to do. The problem is that the posts (comments) are published under the username of the system user. It’s possible to append some text to the post to indicate which user create the post, but if in the future someone replied to one of those posts, the creator of the post wouldn’t get a notification for it.
One thing you can do with the WP Discourse plugin out of the box is display old WordPress comments on your WordPress site underneath the comments that are pulled from Discourse.