Assuming the list of values for your new partition are not included in any one of your current partitions...
Assuming the values you want to include in your new partition are 'XX', 'YY' and 'ZZ"...

ALTER TABLE table_name
SPLIT PARTITION rest VALUES ('XX', 'YY', 'ZZ')
INTO
( PARTITION new_partition_name
TABLESPACE tablespace_name,
PARTITION rest
STORAGE (PCTINCREASE 0)
)
;