write a component that can store a flag for pinned posts. you can either use existing tables like ossn_entities_metadata and ossn_entities or you could modify the ossn_objects table since that holds most post data.
make the post add a link to newsfeed posts (see the "dislike" component for an example of how to do this) when the logged in user is an admin. use a function like is_AdminLoggedIn() for ease.
modify the OssnWall component to check for this flag and resort the posts with pinned ones at the top.