In a small bowl, whisk together teriyaki sauce, water, sesame oil, brown sugar, ginger paste, red pepper flakes, garlic and pineapple juice.
Put tenderloin pieces in a gallon-sized resealable bag and pour marinade over the top. Seal and let marinate in the refrigerator for at least 2 hours. If using wooden skewers, soak in water for 30 minutes before cooking.
Oil and preheat grill to medium-high heat. After marinating, thread pork and pineapple onto skewers.
Grill skewers for 2-3 minutes on each side, or until pork is cooked through and reaches an internal temperature of 145 degrees F. Remove from grill and serve.