Automatically, from a forum that's not yours? That's the easy way to get your forum taken down. (It's called copyright infringement.)
I don't believe there is any software that takes posts from a forum and migrates them to another one. However, there are programs that take RSS feed information and creates new threads for each entry. If you look on this forum's index page you will notice the RSS icon next to each forum. You could use that RSS feed to fetch new posts, and use the software to create threads on your forum based on the RSS feed. These scripts are useful if you want to include news feeds on your forum. But they would not be useful to just copy all of a forums posts.