Derivation rules are used for entities.
For CR fields, you can do that by enhancing the CR GenIL model (transaction GENIL_MODEL_BROWSER). Create an enhancement (for example, called ZCR) and change the CR Reason attribute to mandatory. Another way is to find the handler class defined for the CR GenIL model. Enhance the get_attribute_properties method (using a post-exit) to change the CR reason to mandatory in the code.