Dan Chisena has made the switch to safety and Marcus Allen is getting work at inside linebacker.
According to The Athletic, Iowa, Nebraska, and Ohio State dissented.
Thompkins will join fellow Nittany Lion Saeed Blacknall in the Steelers' receiving corps.
Clifford hauled in nine catches for 125 yards in a tough 30-24 overtime loss to Brownsburg on ESPN.
Blacknall has re-signed with the Steelers after being released by the organization one week ago.
The Big Ten is considering playing a schedule that would start the week of Thanksgiving, per a report by the Milwaukee Journal Sentinel's Jeff Potrykus.