The name of a rice-cooker may imply that it's meant to cook only one thing; However, all a rice cooker does is boil a liquid until it reaches a set temperature, then keep the contents warm until you tell it otherwise. This is essentially the same process for making many soups. Because of the heat containment, they are especially good for cooking summer soups because they won't heat the area as much as cooking on a stove.
Understand how your rice cooker works. Simple models have simple, single-button controls. Upscale models are programmable with multiple settings. Either is fine for making soups -- just know how to use it before you start cooking.
Check the capacity of your rice cooker against the yield of your recipe. Many rice cookers are smaller than the average volume of a soup recipe. If necessary, cut your ingredient amounts in half.
Precook all meats you want to use in your rice cooker. The cooking time for the cooker itself may not be long enough to fully cook your meat. Though not vital for food safety, you may want to do the same with crunchy vegetables like peppers, carrots and celery.
Mix all ingredients in another container before pouring into your rice cooker. This will prevent splashing over the side of the smaller rice cooker basin.
Start your rice cooker with a low-temperature cook setting -- or just "Cook" in simpler models. It will automatically shift to a warm simmer when the soup reaches the right temperature.