Adding subpartitions to existing partitioned table In Oracle 11g, is there a way to introduce subpartitions into an existing partitioned table?

I cannot seem to find a combination of EXCHANGE and SPLIT partition that does the right thing.  SPLIT will split a partition into multiple partitions, not introduce subpartitions.  Any suggestions?

I did find an existing post on partitioning an existing non-partitioned table by exchanging to a table with one partition and then using SPLIT, but can't figure out the equivalent process for subpartitions.
A new subpartition can be added to the existing partitioned table using the following command:

ALTER TABLE PART_TEST
modify partition OCT19 
add subpartition OCT19AXCS 
values ('AXCS');