There's two common ways to do this - using either Python or Modelbuilder. I'm not great at Python, so I will a discussion of how to do it with that to others. As for Modelbuilder, here's what you could do:
1. Place copies of all the feature classes in one folder, preferably with a short folder chain to the root.
2. Design a Model using Modelbuilder with the following components:
- Iterate ______ (Modelbuilder-only tool) - this will loop through all the feature classes to run the following processes. Choose the appropriate Iterator to match your data.
- Add Field geoprocessing tool
- Calculate Field geoprocessing tool
Note that the Calculate field would use the Value or Name output from the Iterate to populate the file name via the use of a
Inline Variable Substitution.
What is ModelBuilder?—Help | ArcGIS for Desktop
ArcGIS Help (10.2, 10.2.1, and 10.2.2) - Iterator Examples
ArcGIS Help (10.2, 10.2.1, and 10.2.2) - Inline Variable Substitution
After the model is run and all the feature classes now have the new field with the calculated value, you can run a Union geoprocessing tool to combine them.
Chris Donohue, GISP