Actually, FFT and Inverse FFT are essentially the same algorithms, so the second FFT is really an IFFT, converting the spectrum back into a time-domain waveform.
I expect that the .imp file is the same sort of binary file as an .fft or .raw file.? It should have a readable header at the top that lists the LTspice settings and the signal name, followed by binary data for the remainder of the file.
Obviously it has nothing whatsoever to do with Adobe Audition, but that was the result?because it may be the most common use of ".imp" as a filename's extension.? The web search doesn't check what is actually inside the file.? Nobody actually "owns" any filename extension.
Andy
