WPMU DEV GLOBAL AUTHOR POSTS FEED Plugin
Create a global feed of posts, for any single author, from across an entire Multisite network.
WPMU DEV GLOBAL AUTHOR POSTS FEED Plugin grabs all the posts from a single author, from across an entire Multisite or BuddyPress network, and puts them in a single feed.
A feed just for a selected author that combines all their posts into one big RSS job.
Please visit official plugin page for more information