ORA-01839: date not valid for month specified (to_yminterval function)

There is nothing wrong with your sql.  What version of 9 are you using?  I would suggest patching to the latest version. It appears to be a bug.

SQL> select * from v$version;

BANNER
--------------------------------------------------------------
Oracle9i Enterprise Edition Release 9.2.0.1.0 - Production
PL/SQL Release 9.2.0.1.0 - Production
CORE    9.2.0.1.0       Production
TNS for 32-bit Windows: Version 9.2.0.1.0 - Production
NLSRTL Version 9.2.0.1.0 - Production

You are correct, by excluding the two hiredates that will fall in Feb with the to_yminterval function applied, I do not get the error.  Below is the successful query excluding the enames with the problem hiredates and then a query showing  the actual hiredates:

SQL> select ename, hiredate, hiredate + to_yminterval('01-02')
  2  from emp
  3  where ename not in ('JONES', 'SOYUHU');

ENAME      HIREDATE  HIREDATE+
---------- --------- ---------
SMITH      17-DEC-80 17-FEB-82
ALLEN      20-FEB-81 20-APR-82
WARD       22-FEB-81 22-APR-82
MARTIN     28-SEP-81 28-NOV-82
.
.
.SQL> select ename, hiredate
  2  from emp
  3  where ename in ('JONES', 'SOYUHU');

ENAME      HIREDATE
---------- ---------
JONES      30-DEC-99
SOYUHU     30-DEC-99