BuddyPress.org

For #2, you could create a profile field for a user to input their existing blog URL. But this would only be a link to that person’s blog.

I’m guessing you want to import a user’s external blog based solely on the blog URL; this isn’t possible. However, something could be done with a blog’s RSS feed, though like I said before such a plugin doesn’t currently exist (that I’m aware of!).