How to make an existing column autoincrement in Oracle
If you insist on using IDENTITY, it is not be possible  - you cannot modify non-identity column to identity.  However,  Oracle lets you use sequence.nextval as default, so you may get similar functionality  : 

CREATE SEQUENCE SEQ_USER START WITH 1000; --assuming max(USERS.user_id) =999
ALTER TABLE USERS MODIFY (USER_ID DEFAULT SEQ_USER.NEXTVAL); 


It will work as GENERATED BY DEFAULT AS IDENTITY.