MIX pretzel crumbs, sugar and butter; press onto bottom and up side of 9-inch pie plate. Refrigerate until ready to fill.
MIX next 3 ingredients in large bowl. Whisk in 2-1/2 cups COOL WHIP; pour into crust.
FREEZE 6 hours. Remove from freezer 15 min. before serving; let stand at room temperature until slightly softened. Top with remaining COOL WHIP.