Well, unfortunately you can’t grill in your slow cooker, but you can use a cooking rack!
Slow-cooking and grilling are two quite different ways of cooking. Grilling involves high heat applied directly to food. Slow cooking involves low, moist heat surrounding food. You could put a rack or a trivet into the bottom of a slow cooker and put food on top of it, either directly or in a container. But the outcome won’t be grilled food; it will be slow-cooked food.
Hope this is helpful!