Lockpicking.  We may need some time inside, and intelegent undead will note the change in the door's condition. With a large sized greatsword, Kasumi should be able to destroy any single, normal Zombie in one blow... Then Sam will tap her with the wand, leaving the other Zombies without even the means to wonder what just happened.  That should hopefully give Ithi'ka or Garret time to open the door.