This feature is not available as far as I an aware.

You can however work around it to some extent. What I have done is created a category called Announcements and when I post a message as the admin using this category, I then lock it so users cannot reply to this message. Occasionally a user posts a message using this category, I then edit the message the user has posted and change the category to one of the other categories that we have available.

David Watkins