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));

string newGDDItemPath = @"C:\path\ArcGIS\Project\NewLocation.gdb";
string oldGDBItemPath = @"C:\Path\Project\OldLocation.gdb";

//Create a new GDB item and add it to the project
var newGDBItem = ItemFactory.Instance.Create(newGDDItemPath) as IProjectItem;
if (newGDBItem == null)
  return;
var success = Project.Current.AddItem(newGDBItem);
//make the newly added GDB item the default
if (success)
  Project.Current.SetDefaultGeoDatabasePath(newGDDItemPath);
//Now remove the old item
var oldGDBItem = Project.Current.GetItems<Item>().FirstOrDefault(i => i.Path == oldGDBItemPath) as IProjectItem;
if (oldGDBItem == null)
  return;
var removeSuccess = Project.Current.RemoveItem(oldGDBItem);

//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;
});

// Get layout Template Path from the project's home folder and combine it with a file name
var projectPath = CoreModule.CurrentProject.HomeFolderPath;
var layoutTemplateFilePath = System.IO.Path.Combine(projectPath, templateFileName);
// Create a new layout project item with the layout file path
await QueuedTask.Run(() =>
{
  // Create an IProjectItem using a layout template pagx file
  IProjectItem pagx = ItemFactory.Instance.Create(layoutTemplateFilePath) as IProjectItem;
  // Add the IProjectItem to the current project
  Project.Current.AddItem(pagx);
});

//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);
});