Item Create( 
   string uri,
   ItemFactory.ItemType itemType
)

//Adding a folder connection
string folderPath = "@C:\\myDataFolder";
var folder = await QueuedTask.Run(() =>
{
  //Create the folder connection project item
  var item = ItemFactory.Instance.Create(folderPath) as IProjectItem;
  //If it is succesfully added to the project, return it otherwise null
  return Project.Current.AddItem(item) ? item as FolderConnectionProjectItem : null;
});

//Adding a Geodatabase:
string gdbPath = "@C:\\myDataFolder\\myData.gdb";
var newlyAddedGDB = await QueuedTask.Run(() =>
{
  //Create the File GDB project item
  var item = ItemFactory.Instance.Create(gdbPath) as IProjectItem;
  //If it is succesfully added to the project, return it otherwise null
  return Project.Current.AddItem(item) ? item as GDBProjectItem : null;
});