Yes, you can make the prosciutto and pear salad ahead of time, but to ensure freshness, it is best to store each ingredient separately. You can make the dressing ahead of time, but store it separately as well to prevent the ingredients from going bad. You might also want to wait to cut the pears until you are ready to assemble the salad. To prevent the pears from turning brown, you can toss them with lemon juice. Be sure to assemble and dress the salad just before serving.