Enable Sitewide Widgets for WordPress Multisite
The MUWidgets plugin makes this possible. It extends the standard WordPress widgets to be able to run on another site within the network.
Install the plugin and you’ll get a number of new widgets prefixed with the word ‘Site’. As admin, you’ll be able to select any site within your network and click ‘save’. Now the widget is operating as if it was on the other site. You can use this for displaying the blogroll, recent posts from the main blog, or alerts and messages across the network.
Note: When we last checked, this plugin above was outdated (“This plugin hasn’t been tested with the latest 3 major releases of WordPress.”). However, we found an article that provides a solution here: WordPress Multisite and Sharing Widgets.