Yes, you can derive a form from another form (as long as it is not marked as sealed). How? Exactly the same way as you do with any other class.
Suppose you start with EntityForm and then you want to create form ExtendedEntityForm which adds, say, a couple of controls to the EntityForm. To do this, you just have to do the following:
public class EntityForm: Form
public class ExtendedEntityForm: EntityForm