ActivityFeed service exposes PublishUpdate method which allows you to add new update.

Update PublishUpdate(UpdateContent content, 
    string publisherId, 
    string source, 
    IEnumerable<Topic> topics, 
    bool isPublic, 
    IEnumerable<UserGroup> userGroups)
Parameters description: Note: Custom content of update required custom update template.

When post new status update, Updates Module executes PublishUpdate method.

activityFeedService.PublishUpdate(new Post(dto.Text), identityProvider.Identity.Name,
  dto.Source, newTopics, true, Enumerable.Empty<UserGroup>());
There are two extension methods that allow you to publish public updates without setting permissions.

public static Update PublishPublicUpdate(this IActivityFeedService service, UpdateContent content, string publisherId, string source, params Topic[] topics)

        public static Update PublishPublicUpdate(this IActivityFeedService service, UpdateContent content, string publisherId, string source, IEnumerable<Topic> topics)