The export function is for moving all contents and cannot be used to move a single post. Yes, it's copy and paste. Alternatively the other blogger could email you a copy of the article.
Are you and the original author aware of the impact and penalty that can result from creating duplicate content? Search engine aim to provide the most relevant content to those who are searching. Creating duplicate content simply pollutes the blogopshere and dilutes the relevance of the search results. Consequently, the blog where the post was originally published and the blog where the content is duplicated can be penalized.
My suggestion is that you post an only an excerpt with proper attribution followed by a "read the full article link" to the original post on the original blog it was published on.