BuddyPress Feeds plugin gives ability to define a custom feed that will be imported right in the global activity stream (or elsewhere as per extensions).
Users can define their their feeds to display content from other sites. Plugin honors the date items were published in the external feed and import them on the same date in local activity stream.
Plugin requirements:
/wp-content/uploads/ directory, that is properly registered if the path was changed (for storing extra plugin's data)