Here's a little shortcut. You can calculate extents in the Field Calculator dialog using:
dim Output as double dim pGeom as IGeometry set pGeom = [shape] Output = pGeom.Envelope.XMin
Choose advanced, paste what is above and set your field equal to Output.
NOTE: If you want to add the values in Decimal Degrees or Web Mercator there's an app for that. Ask Tim Williams how to install his Calculate Extent button.