Click the Settings Cog.
Select Stream Administration under Posts.
Select multiple posts.
Click the caret on the Bulk Actions Button.
Select Add Tags (#).
Begin typing to search for an existing hashtag and then select it from the dropdown, or type a new hashtag and hit Enter.
Repeat as desired (to apply multiple hashtags) and click Next.
Confirm by clicking Add Tag(s).
Note: The bulk Add Tags feature will not trigger email notifications for users who are following the updated posts or tags.