AddBehaviorExtensions_full.cs

// Get the machine.config file.
Configuration machineConfig = ConfigurationManager.OpenMachineConfiguration();
// Get the machine.config file path.
ConfigurationFileMap configFile = new ConfigurationFileMap(machineConfig.FilePath);

// Map the application configuration file to the machine configuration file.
Configuration config = ConfigurationManager.OpenMappedMachineConfiguration(configFile);

ConfigurationSectionGroup svcModel = config.SectionGroups.Get("system.serviceModel");
ExtensionsSection extensions = (ExtensionsSection) svcModel.Sections.Get("extensions");

ExtensionElement e = new ExtensionElement("MyName", "MyType, MyNamespace.MyType, Version=1.0.0.0, Culture=neutral, PublicKeyToken=4275f802b89cdd22");
extensions.BehaviorExtensions.Add(e);
extensions.SectionInformation.ForceSave = true;

config.Save(ConfigurationSaveMode.Full);