If you wanted to do hexadecimal conversion, it would be easy since Oracle supplies a number format for this. The command would be something like
select to_char(1234567890,'XXXXXXXXXX') from dual;
I'm fairly sure that for base 64 you will have to write your own conversion function. Fortunately the algorithm to do this is simple. Here is some pseudo-code that should be fairly close to what you need, at least for converting positive integers.
begin
base_64_str := ''
n := {binary number to be converted}
while n <> 0
begin
base_64_str := '<' || mod(n,64) || '>' || base_64_str
n := trunc(n/64)
end
return base_64_str