Yes, you can. Beside each product image, you will see the pencil icon. Simply click that icon and add custom alt text for the selected photo.
Please note that if you change the format of your alt text, all manually edited alt text will be locked. The new alt text format will only apply to the ones you have manually edited.
If you want to apply the new alt text format to custom alt text, click the "lock icon". This will unlock the format for that image and the new alt text format will be applied.