@Luis Perhaps I misunderstood. I know you want to assign the external ID to an Item record, but are you saying that you want to assign an external ID to segments? I thought you wanted to assign segments to item records. Can you please clarify.

We want to assign External IDs to Items, Custom Segment Values and Classes. The External IDs will just be the same with the Item Name, Class Name and Custom Segment Value Name.

If I understand correctly, you want a script that will set the external id of every created/edited record to the same value as the record name. It can be done via a trivial user event script for all record types that support one, and possibly even with a workflow.

And depending on your skill level that should take < 1 hour to set up