public class ImportInventory
{ 
	public const string HardwareInventoryReportID = "{00000000-0000-0000-0000-000000000001}";
	static void Main(string[] args)
	{
		if (args != null && args.Length >= 2)
		{
			string fileName = args[0];
			string siteCode = args[1];
			ImportInventoryReport(siteCode, fileName);
	}
		else
		{
			Console.WriteLine("Usage: InventoryImportExample <MofFileName> <site code>");
	}

		Console.WriteLine("Press any key to exit");
		Console.ReadLine();
}

	public static void ImportInventoryReport(string siteCode, string fileName, 
		string inventoryReportID = HardwareInventoryReportID, 
		InventoryImportType importOption = InventoryImportType.BothClassAndReport)
	{
		if (File.Exists(fileName)==false)
		{
			throw new FileNotFoundException("MOF file not found", fileName);
	}

		string mofToImport = File.ReadAllText(fileName);

		// Get the SMS_InventoryReport class.
		try
		{
			string scope = string.Format(@"root\sms\site_{0}",siteCode);
			ManagementClass cls = new ManagementClass(scope, "SMS_InventoryReport", null);

			ManagementBaseObject inParams = cls.GetMethodParameters("ImportInventoryReport");
			inParams["InventoryReportID"] = inventoryReportID;
			inParams["ImportType"] = (uint)importOption;
			inParams["MofBuffer"] = mofToImport;
			ManagementBaseObject retVal = cls.InvokeMethod("ImportInventoryReport", inParams, null);

			// Get current site code.
			uint resultCode = (uint)retVal["StatusCode"];

			if (resultCode == 0)
			{
				Console.WriteLine("ImportInventoryReport for file {0} succeed ", fileName);
		}
			else
			{
				Console.WriteLine("ImportInventoryReport for file {0} failed with error code:{1} ", fileName,resultCode);
		}
	}
		catch (ManagementException e)
		{
			Console.WriteLine("Failed to execute method ImportInventoryReport for file {0}: {1}", fileName, e.ToString());
	}
}

	public enum InventoryImportType
	{
		ClassOnly = 1, 
		ReportOnly = 2, 
		BothClassAndReport = 3
}
}

An example MOF file.
================================

[ SMS_Report (TRUE),
  SMS_Group_Name ("User Account"),
  SMS_Class_ID ("MICROSOFT|USER_ACCOUNT|1.0"),
  Namespace ("root\\\\cimv2") ]
class Win32_UserAccount : SMS_Class_Template
{
	[ SMS_Report (TRUE), key ]
	String	 Domain;
	[ SMS_Report (TRUE), key ]
	String	 Name;
	[ SMS_Report (TRUE) ]
	UInt32	 AccountType;
	[ SMS_Report (TRUE) ]
	String	 Caption;
	[ SMS_Report (TRUE) ]
	String	 Description;
	[ SMS_Report (TRUE) ]
	Boolean	 Disabled;
	[ SMS_Report (TRUE) ]
	String	 FullName;
	[ SMS_Report (TRUE) ]
	DateTime	 InstallDate;
	[ SMS_Report (TRUE) ]
	Boolean	 LocalAccount;
	[ SMS_Report (TRUE) ]
	Boolean	 Lockout;
	[ SMS_Report (TRUE) ]
	Boolean	 PasswordChangeable;
	[ SMS_Report (TRUE) ]
	Boolean	 PasswordExpires;
	[ SMS_Report (TRUE) ]
	Boolean	 PasswordRequired;
	[ SMS_Report (TRUE) ]
	String	 SID;
	[ SMS_Report (TRUE) ]
	UInt8	 SIDType;
	[ SMS_Report (TRUE) ]
	String	 Status;
};