public bool AddItem( 
   IProjectItem item
)

/// Add a folder connection to a project
Item folderToAdd = ItemFactory.Instance.Create(@"C:\Data\Oregon\Counties\Streets");
bool wasAdded = await QueuedTask.Run(() => Project.Current.AddItem(folderToAdd as IProjectItem));

/// Add a file geodatabase or a SQLite or enterprise database connection to a project
Item gdbToAdd = folderToAdd.GetItems().FirstOrDefault(folderItem => folderItem.Name.Equals("CountyData.gdb"));
var addedGeodatabase = await QueuedTask.Run(() => Project.Current.AddItem(gdbToAdd as IProjectItem));

/// Import a mxd
Item mxdToImport = ItemFactory.Instance.Create(@"C:\Projects\RegionalSurvey\LatestResults.mxd");
var addedMxd = await QueuedTask.Run(
              () => Project.Current.AddItem(mxdToImport as IProjectItem));

/// Add map package      
Item mapPackageToAdd = ItemFactory.Instance.Create(@"c:\Data\Map.mpkx");
var addedMapPackage = await QueuedTask.Run(
              () => Project.Current.AddItem(mapPackageToAdd as IProjectItem));

/// Add an exported Pro map
Item proMapToAdd = ItemFactory.Instance.Create(@"C:\ExportedMaps\Election\Districts.mapx");
var addedMapProjectItem = await QueuedTask.Run(
              () => Project.Current.AddItem(proMapToAdd as IProjectItem));

//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 successfully 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 successfully added to the project, return it otherwise null
  return Project.Current.AddItem(item) ? item as GDBProjectItem : null;
});

//Import a pagx into a project.

//Create a layout project item from importing a pagx file
await QueuedTask.Run(() =>
{
  IProjectItem pagx = ItemFactory.Instance.Create(
                            @"C:\Temp\Layout.pagx") as IProjectItem;
  Project.Current.AddItem(pagx);
});