Desrosiers was selected by the Stars in the second round (No. 54) of the 2013 NHL Draft and finished his third season with the Rimouski Oceanic of the Quebec Major Junior Hockey League.
This season, the 6-foot-1, 195-pound 18-year-old had the fourth-lowest goals-against average in the QMJHL (2.65), was tied for the sixth-highest save percentage (.907), ranked fifth in wins with a 31-14-7 record, and had five shutouts.
Desrosiers, a native of Saint-Hyacinthe, Quebec, had a 7-2-1 record with a 2.34 GAA and a .917 save percentage in 11 playoff games.
Other signings Saturday:
Dotchin, 20, scored 11 goals and 36 points this season for the Barrie Colts of the Ontario Hockey League. He led the Colts in penalty minutes with 121 and was third with a plus-22 rating. The Cambridge, Ontario native has appeared in 11 playoff games for the Colts, with three goals and five points.
Dotchin, 6-3, 209, has played in 193 OHL games over his three-year junior career, scoring 18 goals and 77 points with 279 penalty minutes. He set career-highs in 2013-14 in goals (11), assists (25) and points (36).
Dotchin was a sixth-round pick (No. 161) of the Lightning in 2012.
Helgesen, 20, scored 10 goals and 51 points with a plus-31 rating and 67 penalty minutes in 71 games this season for the Calgary Hitmen of the Western Hockey League. He set WHL career highs in all offensive categories, including goals, assists, points, power-play goals (4) and plus/minus. The Grand Prairie, Alberta native also appeared in six WHL playoff games this spring, with a goal and an assist.
A seventh-round selection (No. 187) of the Ducks in 2012, Helgesen has career totals of 13 goals and 85 points with a plus-60 rating and 246 penalty minutes in 199 WHL games. He has also appeared in 14 career WHL Playoff games, posting one goal and two assists.