Is it okay to slow cook sausage casserole without pan frying the sausages?
There's no health issue here, the sausages will be cooked enough to be safe. The reason you fry off the sausages first is that you make the casings more edible, get flavor from maillard reactions and browning, and maybe get rid of some of the fat (if you discard the fat that comes out of the sausages that is). 

I'm thinking that the sausage casings could end up being a bit soggy, however that depends on several factors and you'll probably get a good result. Worst case is you don't eat the sausage casings.