Yes, you are right. There is no MACRO for Modemoff bit configuration, but you can call API function (
phStatus_t phhalHw_Rc663_WriteRegister(phhalHw_Rc663_DataParams_t * pDataParams, /**< [In] Pointer to this layer's parameter structure*/
uint8_t bAddress, /**< [In] Register Address. */
uint8_t bValue /**< [In] Register Value. */
);
to configurate your application.