BuddyPress.org

Best I can tell looking for 30 seconds is that there is no backwards import.

There’s two places where BP adds users to blogs…

add_action( 'wpmu_new_blog', 'bp_blogs_record_blog', 10, 2 );

…and…

add_action( 'add_user_to_blog', 'bp_blogs_add_user_to_blog', 10, 3 );

So only when a new blog is created, or a user is added to a blog…

I’ll look into this more this weekend if no one else chimes in with a better explanation.

The tables to manually edit would be wp_bp_user_blogs, wp_bp_user_blogmeta, wp_bp_user_blogs_comments, and wp_bp_user_blogs_posts…

If there really is no import script for previous blogs, add a ticket to the trac as an enhancement request and we’ll see about putting it in. (it may already be there but I’m just too tired to see it at the moment?)