Preheat the slow cooker if necessary; see the manufacturer's instructions. Pour the cranberry and raspberry drink into the slow cooker pot. Add the frozen fruits, the sugar and crème de cassis, if using. Stir together, then add the star anise and halved cinnamon stick.
Cover with the lid and cook on high for 1 hour. Reduce the heat and cook on low for 2–3 hours, or set to auto for 3–4 hours, until piping hot.
Strain, if liked, then put the star anise and cinnamon into small heatproof glasses. Ladle the hot punch into the glasses and add a few fresh raspberries, if liked.