Only one I know for sure was the old man at Gettysburg who fought for the Union. Can't recall his name.
Actually there were several civilians who joined the armies during the Gettysburg campaign, among them:
John Burns, who became famous, supplied his own gun and was allowed to fight in the
Union ranks; wounded.
J. F. McKenrick, a young Pennsylvania lad, was allowed to serve with the medical staff in
Confederate ranks, no gun.
Charles Francis Wheatley, was accepted into the
Union ranks (12 Mass.) and supplied with a gun; wounded.
Unidentified, man of African descent, who joined during the battle and fought within the
Union ranks (5 Ohio).
Unidentified, older man, joined Lang's Florida brigade,
Confederate, on the march, but dropped out after one day.
Civilian volunteers seem to have had little trouble convincing their military brethren of their sincerity; I suppose the fact that they were willing to risk death on the battlefield was considered sufficient proof of their loyalty.
In addition, a few civilians were officially enrolled in the Confederate army as it marched northward through Maryland.