In theory, a satellite dish can be used as high gain aerials for purposes other than satellite TV, but it depends on a lot on the frequencies you want to pick up. Unfortunately, FM has a such a long wavelength that you would need an enormous satellite dish for it to effectively concentrate the FM signal. You would be much better off with a proper outdoor FM aerial. Due to their long wave length, it's worth noting that FM aerials are rather long and directional FM aerials are larger than high gain TV / Band III aerials. See this page for some info.
Typically the most common hack for old satellite dishes is to use them as ultra high Wi-Fi antennas to pick up Wi-Fi reception from several miles away, since Wi-Fi operates at 2.4GHz or 5GHz. Basically, what this hack involves is replacing the LNB with a Wi-Fi aerial and then pointing the dish at a very low angle such that it picks up its signal from a horizontal direction.