public bool CanAddAsFavorite( 
   Item item
)

{
  var itemFolder = ItemFactory.Instance.Create(@"d:\data");

  // is the folder item already a favorite?
  var fav = FavoritesManager.Current.GetFavorite(itemFolder);
  if (fav == null)
  {
    if (FavoritesManager.Current.CanAddAsFavorite(itemFolder))
    {
      fav = FavoritesManager.Current.AddFavorite(itemFolder);
    }
  }
}

{
  gdbPath = "@C:\\myDataFolder\\myData.gdb";
  var itemGDB = ItemFactory.Instance.Create(gdbPath);

  // is the item already a favorite?
  var favorite = FavoritesManager.Current.GetFavorite(itemGDB);
  // no; add it with IsAddedToAllNewProjects set to true
  if (favorite != null)
  {
    if (FavoritesManager.Current.CanAddAsFavorite(itemGDB))
      FavoritesManager.Current.InsertFavorite(itemGDB, 1, true);
  }
}

{
  StyleProjectItem styleItem = Project.Current.GetItems<StyleProjectItem>().
                          FirstOrDefault(style => style.Name == "ArcGIS 3D");
  if (FavoritesManager.Current.CanAddAsFavorite(styleItem))
  {
    // add to favorites with IsAddedToAllNewProjects set to false
    FavoritesManager.Current.AddFavorite(styleItem);
  }
}