In a new GP 2013 Feature of the Day, Microsoft shows off Custom Fields in Advanced HR
Feature of the Day: Custom Fields in Advanced HR – Inside Microsoft Dynamics GP – Site Home – MSDN Blogs
Payroll & HRTips & Hidden FeaturesMicrosoft & the GP EcosystemGP 2013
Archived post from 2012. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.