You can put any raw meat in the slow cooker, that hasn't been browned first. However, when meat isn't browned first the flavour is less as there is no caramelisation. The slow cooker won't brown the meat, so it will be pale and unappealing. And, adding cold ingredients to the slow cooker means that it can't start cooking straight away, as it will take time to get the temperature up to the correct cooking point. For best results, and if you have time, it's better to brown the meat first.