Illinois provides special admission rules for public defenders licensed in other jurisdictions under the Illinois Legal Services Organization Rules. The Illinois Pro Bono/Emeritus Rules allow attorneys to provide pro bono services under a government entity.
Illinois also provides an Illinois Multijurisdictional Practice Rule (MJP) within its rules of professional conduct. Comment 16 to that rule allows for an interpretation that exempts government attorneys from the unauthorized practice of law.
Please also see the Illinois Bar Reciprocity page to view the requirements for Admission on Motion, and the Illinois Pro Hac Vice page to view the requirements for admission to a particular proceeding.