Edit The Field Defination Of A Work Item Type

Hi

My Question Is About The Tfs Work Items .. Is It Supported By The Tfs Api To Edit The Work Item Type Fields Definitions ?
So I Need To Make What Descriped In That Link (https://msdn.microsoft.com/en-us/library/ms400654(v=vs.110).aspx) But By The C# Tfs Api.

Clearfication 1 : In The "Bug" Work Item Type I Created A Defination Called "Customer" .. And Added Its Value Items By Wizard 

.. So My Question Is It Supported By The C# Tfs Api To Edit Or Delete Items To That Field ?

Clearfication 2 : I Can Retrieve That Values By
AllowedValuesCollection Collec = WorkItemStore.FieldDefinitions["Customer"].AllowedValues;
But The Allowed Values Is Readonly Property Without Any Public Setter.

Thanks In Advance.