SQL & PL/SQL :: How To Add Column In Unique Index

I have created one unique index on one column of my table. Now i would like to add one more column in the same index without dropping the index.

SQL > CREATE TABLE DEBUG_TABLE
2 (
3 SLNO NUMBER,
4 MESSAGE VARCHAR2(4000 BYTE),
5 CREATED_DATE DATE DEFAULT SYSDATE,
6 CREATED_TIME TIMESTAMP(6) DEFAULT SYSDATE
7 );

Table created.

SQL > CREATE UNIQUE INDEX index_debug1 ON debug_table (SLNO);

Index created.

SQL > ALTER INDEX index_debug1 ADD COLUMN MESSAGE;
ALTER INDEX index_debug1 ADD COLUMN MESSAGE
*
ERROR at line 1:
ORA-02243: invalid ALTER INDEX or ALTER MATERIALIZED VIEW option

SQL >