declare
    object_exists exception;
    except_00957  exception;
    pragma exception_init (object_exists , -01430);
    pragma exception_init (except_00957 , -00957);
begin
   execute immediate 'ALTER TABLE GP_PRICING ADD (VALOREXTRA NUMBER(7,2) )'; 
   exception 
       when object_exists then null;
       when except_00957 then null;
end;