You can't perform an inverse FFT on the data that BASS gives you (it only gives the amplitudes), but I don't think that would be much use to you anyway.

If you want to use FFT to modify the sample data, you should use a DSP callback function, in which you would perform an FFT, process the FFT output, and then perform an inverse FFT. You'll probably want to overlap the FFTs, to avoid "clicks" between the blocks of processed data (look into the "overlap-add" and "overlap-save" techniques). Note this is not particularly simple stuff