Is it possible to make a comment out of a post that already exists (with the same name). For example, if the post 'Hello World' exists and a users posts another 'Hello World' post, it should make the second post a comment beneath the first post... How can I do this?
You can use get_page_by_title http://codex.wordpress.org/Function_Reference/get_page_by_title to check if another page/post is using the same title. If so, get the content of that post and insert it using wp_insert_comment, and delete the original post with wp_delete_post. Finally, glue all these together inside the save_posts action hook.