SQL & PL/SQL

CREATE TABLE sk_abc_123 
  ( 
     a NUMBER, 
     b NUMBER 
  ); 

INSERT INTO sk_abc_123 
VALUES      (1, 2); 

INSERT INTO sk_abc_123 
VALUES      (10, 20); 


CREATE INDEX sk_abc_123_id1 ON sk_abc_123(a); 

now I want to have index on (a,b) columns combinedly as per my requirement, the one possible way is 

DROP INDEX sk_abc_123_id1;
CREATE INDEX sk_abc_123_id1 ON sk_abc_123(a,b); 

Can't we do it with out dropping an index? Meaning can't we alter an index to add a new column to the existing index?