Michigan does not provide special admission rules for government attorneys licensed in other jurisdictions.
However, Michigan provides a Michigan 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 Michigan Bar Reciprocity page to view the requirements for Admission on Motion, and the Michigan Pro Hac Vice page to view the requirements for admission to a particular proceeding.