Adding instruments that are not in the instrument list

Hi there! I wanted to practice writing for an orchestra, I chose Schindler's List (John Williams) to practice on, unfortunately I couldn't find a French Horn (possibly Flugel horn) or a Cor Anglais, which I would really love to be added to the instrument list as it isn't as piercing as the Oboe, even though I do like the oboe as well. Does anyone know how I could do this. Much appreciated.