There is also no point in using 64 bit audio files. Even 32 bit floating point is already overkill. The best choice for recording is 24 bit, for internal processing 64 bit floating point does make sense, but not for storing.
In any case, your mac comes with a very capable audio file format converting utility that can be used from the command line and reads/writes practically any audio file format you will encounter, including 64 bit wav. Type "afconvert" on your command line in terminal.app to see the options it offers. If you're not used to using the command line you might find this very uncomfortable, but it's really worth getting into it.
Let me know if you need help,
Jazz