Hossack scored in the 15th minute of the extra period and Austin Shanks added an insurance goal one minute later as the Warriors took the best-of-five series for Canada's box lacrosse junior championship 3-1.
Dan Lintner and Curtis Knight both added two goals for the Warriors, while Anthony Malcolm, Jeremy Bosher, and Quinn Smith each scored twice for the Salmonbellies.
Penalties proved costly for New Westminster as Whitby scored five power-play goals.
The Warriors held the Salmonbellies to just one goal in the second period, but New Westminster rallied in the third, scoring three goals in less than two minutes to take a 6-5 lead.
Whitby struck back with a goal by Reilly O'Connor to tie the game, and the back-and-forth battle ensued as the Salmonbellies regained the lead and lost it twice more.
Warrios goalie Alexis Buque made 51 saves while his Salmonbellies counterpart Frank Scigliano stopped 53 shots in the losing effort.