The Fourier block performs a Fourier analysis of the input signal over a running window of one cycle of the fundamental frequency of the signal. The Fourier block can be programmed to calculate the magnitude and phase of the DC component, the fundamental, or any harmonic component of the input signal.
Doing a fft is easier, for a continuous signal, Fourier's method needs infinite sine and cosine terms, rendering the Fourier transform unusable in real situations. Still, i think you can write some codes to do a FT. Doing so, you can use s-functions in simulink.