How to add Subpartition to all partitions
You can just easily loop through the partitions in a PL/SQL loop, and add the subpartitions:

begin
  for p in (select partition_name from user_tab_partitions where table_name = 'PART_TEST')
  loop
    execute immediate 'alter table part_test modify partition ' || p.partition_name || ' add subpartition ' || p.partition_name || 'AXCS values (''AXCS'')'; 
  end loop;
end;
/