Blog aggregating and real-time posting from other blogs on other sites.

Hi. Newbee here.

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How do you set up MU WP to auto import blog posts from specific blogs on other sites into specific corresponding WordPress blog user accounts on a different [multiple user account] site?
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I am trying to figure out how to do this for a new site we are planning. We want to create a site presenting blogs from experts on a particular subject. Of course we would prefer those blogs to reside on our own Multiple User installation of WordPress…

But we realize, practically-speaking, that some of these experts will already have their own blogs and won’t want to start another one with us. But, if they grant us permission to republish their existing blog posts, we want to be able to propagate them over to our site automatically – specifically into a corresponding WordPress user account for each blogger so that they are represented as unique blog posts on our site (“unique” meaning individual, and not necessarily original or exclusive in the blog universe). And, as a unique blog, though imported from another blog source, each post can then be categorized and commented upon as any native blog post could.

How is this done? (thanks in advance!)