The Stars made the moves Saturday before playing at St. Louis.
Fortunus will be available for the 8 p.m. ET game against the Blues. Larsen is flying from Europe to Dallas and will join the team for practice Sunday.
After participating in the Stars' training camp last fall, Larsen had one goal and nine assists in 42 games with Frolunda in the Swedish league. He was a fifth-round pick by Dallas in the 2008 draft.
Fortunus appeared in eight games for Dallas this season. He had 11 goals and 12 assists in 69 games for the Texas Stars.