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);
  }
}

string gdbPath = "@C:\\myDataFolder\\myData.gdb";

var itemGDB = ItemFactory.Instance.Create(gdbPath);

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