How to encrypt external tables in Oracle

Hi Everyone,
We are using Oracle 11.2.0.2.

I knew how to implement encryption of selected columns in Oracle tables.
I read document about modifying the attribute of external table column to create external tables with column encryption. How ever, our external table specifies position of each field in the original external tables. If I modify a few columns, how do I modify the positions of the fields? Here is a external table dll:
create table PION_PARTA_DET_EXT
(
  clmno     VARCHAR2(20),
  prvclmno     VARCHAR2(13),
  clmlnno   VARCHAR2(18),
  memno     VARCHAR2(20),
  nclmtype   VARCHAR2(2),
  clmdatefrom  VARCHAR2(10),
  clmdateto    VARCHAR2(10),
  revcntr   VARCHAR2(4),
  revcntrdt VARCHAR2(10),
  nproc     VARCHAR2(10)
)
organization external