Pour the pineapple juice from the can into a medium bowl and set aside about a cup for the sauce.
Add the frozen meatballs to a 4-6 quart crock pot.
Scatter the pineapple chunks and diced green pepper over the meatballs.
Whisk together the reserved pineapple juice with brown sugar, vinegar, soy sauce, and cornstarch until smooth.
Pour the sauce mixture evenly over the meatballs and cover the crock pot.
Cook on the low setting for 2 hours.
Switch the crock pot to high and continue cooking for 1 more hour to thicken the sauce, which should be bubbling and thick around the edges when done.