Yes, you can most definitely put raw sausages in the slow cooker. However, they will pack in more flavour if seared first.
Searing caramelises the surface which adds a lot of body to the dish. A lot of people believe that searing will also lock the juices into the meat, however, this is not true. So if you are short on time then feel free to skip the pre-sear for the sausages.
The same goes for the onion in this recipe. Pan-frying them first really does add a lot of flavour to the dish. Another thing to note is that it adds more colour to the final result, which is actually pretty important in an otherwise brown and non-colourful dish such as a casserole.