When you are rushing to put ingredients into a slow cooker, it can be tempting to add frozen meat or poultry to the slow cooker so you don’t have to take the time to defrost it. Unfortunately, this is a no go, as when you add frozen meat to a slow cooker, it can take a significant amount of time for that meat to defrost and then get up to a safe cooking temperature.
And not only that, when you add frozen meat or poultry, the temperature of it by the time the end of the cooking time is reached (such as the 4 hour mark on a recipe that says cook on high for 3-4 hours) will not have reached the required end temperature to ensure the meat or poultry is safe to eat.
So you always need to defrost any meat or poultry you add to the slow cooker. So try and remember to take it out to defrost in the refrigerator the night before, or use the defrost function on your microwave right before you add it to the slow cooker.
Sometimes recipes will call for frozen ingredients to be added, often frozen peas and corn. This is safe to do, since the recipe has been designed and tested with those frozen ingredients in mind. Because they are so small, they will be brought up to temperature quickly and not adversely affect the safe cooking of meat or poultry.