Head coach Clay Helton said the Trojans extensively evaluated Schirmer and were left with "no question" that he will be "a great member of our Trojan family."
Schirmer was a 19-year-old player at Mount San Antonio College when he knocked out an official with a punch. Schirmer has said repeatedly that the punch was accidental.
Although Schirmer was arrested after the incident, he did not face criminal charges.
Schirmer will be a redshirt sophomore.