How to alter an existing partitioned table to add sub-partitions to it?
Scenario 1 - redefine the table

Your SQL is adding a sub-partition, not redefining the table.  I suspect you need to redefine the table to have sub-partitions before you can add a sub-partition.

12.2c+

The ability to change a non-partitioned table into a partitioned table via ALTER TABLE was introduced in 12.2c.

Looking at the documentation, it appears that you can change a RANGE partitioned table into a RANGE-LIST partition in the same way.  I haven't tried; be cautious; use DEV for practice.

pre-12.2c

DBMS_REDEFINITION is the de facto way of changing a table between non-partitioned and partition designs.

Google is your friend.  But, here is my recommended link:  

Scenario 2 - convert back to non-partitioned table.

This requirement perplexes me.  Doing such a massive change, multiple times, is probably a Tell-Tale sign that you have a more serious problem that you should be solving.

Any way, I suspect that the only way to do this is with DBMS_REDEFINITION.