Add a new External ID Type in TN 601

How to add a new External ID Type to the existing available list.
The available list has got DUNS, DUNS+4, USer Defined 1 to 3 etc. Now i want to add one type ID to the existing list.
Any help/suggestion is appreciated.

Use the service Be careful about migration from dev to test to prod to make sure the new ID is the same across environments.