declare
    object_exists exception;
    pragma exception_init (object_exists , -01430);
begin
 execute immediate 'ALTER TABLE CAJA ADD (FECVENTADIGITA DATE)'; 
    exception when object_exists then null;
end;
/

declare
    object_exists exception;
    pragma exception_init (object_exists , -01430);
begin
 execute immediate 'ALTER TABLE BOLETO ADD (FECVENTADIGITA DATE)'; 
    exception when object_exists then null;
end;
/

declare
    object_exists exception;
    pragma exception_init (object_exists , -01430);
begin
 execute immediate 'ALTER TABLE DIGITA_ITEM ADD (FECVENTADIGITA DATE)'; 
    exception when object_exists then null;
end;