Yes you can! However, make sure to use hard mozzarella (those that can be grated) and not fresh mozzarella. Adding fresh mozzarella into bechamel sauce will turn the whole sauce grainy! All that hard work in making a smooth bechamel sauce will be ruined! Hard mozzarella however will melt nicely and smoothly. As mentioned above, if you want to use fresh mozzarella cheese, best layer it into the lasagna separately.