Smith Jr., the nation's No. 32-rated DT, selected Michigan State over scholarship offers from Arizona, Cincinnati, Duke, Eastern Michigan, Illinois, Indiana, North Carolina State, Northern Illinois, Notre Dame, Purdue, Syracuse, Tennessee and Toledo.
The 6-foot-2, 276-pound prospect registered a time of 5.21 in the 40-yard dash, 4.49 20-yard shuttle, 26-inch vertical and 74.1 SPARQ at a Nike Combine before his junior season.
The 6-foot-2, 285-pound Allen holds additional scholarship offers from Boston College, Illinois, Indiana, Iowa, Kentucky, Maryland, Missouri, Ole Miss, Purdue and Wisconsin.
Regional powers Michigan and Ohio State also recruited Allen, who will be part of ESPN RecruitingNation's next release of evaluations in June.
The Spartans have six players on board for 2014.