solo cambia el values() por select

CREATE PROCEDURE SP_GENERAR_MORA
AS
INSERT INTO MORA(ID_MORA,NRO_BOLETA,MONTO,MORA_INICIAL,MORA_TOTAL,RESTA,ID_CONCEPTO,ID_ESTUDIANTE)
select '02',0,0.00,1.00,(DATEDIFF(DAY,(SELECT F_PAGO FROM PAGOS WHERE ID_CONCEPTO= 'CP-02'),( SELECT F_PAGO FROM PAGOS_REGISTRADOS WHERE ID_PAGO='P02' ))) * 1, 0,'CP-02','100545'