can we use CASE_N statement to define partion?

Yes. Case statement generally used to handle different cases, if case is true value to be used in SQL statements. same thing can be applied in creation of partion

for example , if empsal <10000 one partion, 10000
create table employee(
empno integer,
ename varchar(20),
esal
)
primary index(empno)
partition by case_n(
empsal<10000,
empsal<20000,
e,psal<30000,
NOCASE,
Unknown
);

)