Code:
SQL>create table test (c1 number) partition by range (c1)
  2  ( partition p10 values less than (10),
  3    partition p20 values less than (20),
  4    partition p30 values less than (30),
  5*   partition pmax values less than (maxvalue))
SQL> /

Table created.

SQL> select high_value from dba_tab_partitions where table_name = 'TEST';

HIGH_VALUE
--------------------------------------------------------------------------------
10
20
30
MAXVALUE

SQL> alter table test add partition p40 values less than (40);
alter table test add partition p40 values less than (40)
                               *
ERROR at line 1:
ORA-14074: partition bound must collate higher than that of the last partition

SQL> alter table test split partition pmax at (40) into (partition p40, partition pmax);

Table altered.

SQL> select high_value from dba_tab_partitions where table_name = 'TEST';

HIGH_VALUE
--------------------------------------------------------------------------------
10
20
30
MAXVALUE
40
HTH