I’m curious as to what you want to do with an FFT. FFT is normally used for visualizing data. The only potential applications I can think of on the H9000 (without a corresponding spectrograph which is not really a real time monitor) is convolution or resynthesis. Convolution would be cool, but I think a dedicated convolution block with some sort of impulse loader would be far more valuable and most of the resynthesis work you can do with bandpass filters and envelope followers.
If you want to build an FFT type of operation in the H9000 simply run a signal through a bank of bandpass filters and then monitor the output of the bandpasses. It essentially does the same thing by binning the input signal into frequency bins. You can then manipulate the frequency bins and remix them to recreate a signal.