How to convert date to timestamp(DD-MON-YYYY HH24:MI:SS.FF format) in oracle?

If I got your question right you need the output in the mentioned Format. That would be a conversion to character

select to_char(cast(sysdate as timestamp),'DD-MON-YYYY HH24:MI:SS.FF') from dual

Of course in the above the FF would also always be 000000

But if you have a timestamp variable you would not cast

select to_char(systimestamp,'DD-MON-YYYY HH24:MI:SS.FF') from dual