declare 
  column_exists exception;
  pragma exception_init (column_exists , -01430);
begin 
  execute immediate 'alter table caja add SERIEIMPFISCALORIGINAL VARCHAR2(6 BYTE)';
  exception when column_exists then null;
end;
/
declare 
  column_exists exception;
  pragma exception_init (column_exists , -01430);
begin 
  execute immediate 'alter table boleto add SERIEIMPFISCALORIGINAL VARCHAR2(6 BYTE)';
  exception when column_exists then null;
end;