Create a package in code

using (new Sitecore.SecurityModel.SecurityDisabler())

      {

        Sitecore.Data.Database db = Sitecore.Configuration.Factory.GetDatabase("master");

        Sitecore.Install.PackageProject document = new Sitecore.Install.PackageProject();

        document.Metadata.PackageName = "Priority3LP";

        document.Metadata.Author = "Olga Kogan";

        Sitecore.Install.Items.ExplicitItemSource source = new Sitecore.Install.Items.ExplicitItemSource(); //Create source – source should be based on BaseSource 

        source.Name = "INSERT SOURCE NAME";

        Sitecore.Data.Items.Item[] items = db.Items.Database.SelectItems("SPECIFY THE SITECORE ITEM PATH TO EXPORT FROM");

        foreach (Sitecore.Data.Items.Item item in items)

        {

          source.Entries.Add(new Sitecore.Install.Items.ItemReference(item.Uri, false).ToString());

        }

        document.Sources.Add(source);

        document.SaveProject = true;

        //path where the zip file package is saved 

        using (Sitecore.Install.Zip.PackageWriter writer = new Sitecore.Install.Zip.PackageWriter("/data/packages/" + "Priority3" + DateTime.Now.Ticks.ToString() + ".zip"))

        {

          Sitecore.Context.SetActiveSite("shell");

          writer.Initialize(Sitecore.Install.Installer.CreateInstallationContext());

          Sitecore.Install.PackageGenerator.GeneratePackage(document, writer);

          Sitecore.Context.SetActiveSite("website");

        }

      }

================

Import a package

1. using (new SecurityDisabler())  
2. {  
3.     using (new ProxyDisabler())  
4.     {  
5.         using (new SyncOperationContext())  
6.         {  
7.             IProcessingContext context = new SimpleProcessingContext();  
8.             IItemInstallerEvents events = new DefaultItemInstallerEvents(new BehaviourOptions(InstallMode.Overwrite, MergeMode.Undefined));  
9.             context.AddAspect(events);  
10.             IFileInstallerEvents events1 = new DefaultFileInstallerEvents(true);  
11.             context.AddAspect(events1);  
12.   
13.             Sitecore.Install.Installer installer = new Sitecore.Install.Installer();  
14.             installer.InstallPackage(MainUtil.MapPath("INSERT PACKAGE PATH INCLUDING THE FILE NAME"), context);  
15.         }  
16.     }  
17. }