It won't look like it was roasted in the oven - however, you could brown it lightly in some olive oil before you place it in the slow cooker. Then it will cook up as pretty as you like! Just keep turning it in the saute pan/skillet until it is golden on all sides. I have one of those slow cookers that I can actually place the "cooker" on the stove - using it to brown the products I'm using. But - in my other ones I just use the skillet/saute pan - not a whole lot to "clean up".
I think it adds to the flavor, also as I always saute my onions, mushrooms, celery, etc. before adding them to the slow cooker. Then I dump in the vegetables - brown whatever meat I'm using - then add the meat/poultry to the bed of vegetables. Yumm!