Can I connect multiple wordpress root installations to one network?

I'm thinking this question is an offshoot of another topic I started earlier here. I am experimenting with a multisite idea I had, with one caveat. It's the caveat that inspires this question, especially after seeing this post.

Let's say there are multiple users each with their own domain and root install of wordpress. Is there any way to set it up so that they could all be connected to a multisite network and still keep their local installs (and databases) intact? I ask because it would involve integrating another application. If something like this IS possible, it would be awesome!