If f0 is 100MHz with +/-100kHz deviation, than we can translate the spectrum around 0Hz, so we will have \$\Delta f = 200kHz\$ which will require a Nyquist sampling at 400 kilosamples/s (400kSa/s).
If I got your question wright, what you are looking for is decimation.
In decimation, if it were to have all the samples taken at 200 MSa/s, than we can simply keep only each 50th sample and discard the other samples. That way we would get the same samples as if it we were sampling at 400 kSa/s (every 50th because 200 MSa/s = 200_000 kSa/s = 50 * 400 kSa/s).
Keep in mind that decimation will fold down other spectral components into the same band, so usually this is done by first applying a low pass filter over all of the high speed 200 MSa/s samples, and only after filtering we can throw away the rest of the samples and keep only each 50th sample after low pass filtering the original high speed signal.
The low pass filter in this numerical example must be at 200kHz, to cut out any higher spectral components. This is necessary only if higher than 200kHz frequencies are present in the FM signal, because by decimation all other multiple of 200kHz bands (e.g. 400kHz-600kHz, or 91.8MHz to 92.0MHz, etc.) will be folded into the first 200kHz, too, and there is no way to distinguish which is which.