SQL & PL/SQL :: Overriding Pre-defined Functions In Oracle

Can we override the pre defined functions in oracle. I am able to do this in my schema. But if DB server contains more than one schema do I need to write the same procedure in all schemes.

we can call the same procedure with schema name from different schemes also . But is there any way to over ride the actual function provided by oracle (just like over riding the equals method of object class in java).

create or replace function to_number(var varchar2) return number
is
no number := var;
begin
return 99999;
end;

SQL> select to_number(' 234 ') oracle_function , dbo.to_number(234) user_function from dual;

ORACLE_FUNCTION USER_FUNCTION
--------------- -------------
234 99999