I wondered what would happen if you tried to do a password expire on an externally identified user so I tried it out on an Oracle 11.2 database. As you might expect, it failed:
SQL> alter user system identified externally
SQL> alter user system password expire
alter user system password expire
ERROR at line 1:
ORA-28010: cannot expire external or global accounts
I guess this is because if an externally identified user had to change his password, he would not be externally identified any longer. Also, a user cannot change itself to or from external identification unless it has the ALTER USER privilege.