Can I mirror blog posts from an external wordpress blog to a local multisite blog?

I’m setting up a multisite blog, but want to offer the ablity of blogger with their own wordpress blogs to set up a blog on the site which would automatically mirror content from their existing wordpress sites. I’ve seen plugins that can mirror sites that exist in the same database, but not mirror external wordpress sites. Does anything that allows this exist? I know I could use RSS, and use some RSS plugin to insert the posts, but I image that would have trouble with plug-in generated content. I’d prefer to have the exact content of the databases mirrored if possible. Thanks.