Well the actual answer is yes, but what will happen is all (or at least 98%) of the existing bacteria and anything else will die. You may get a jump of maybe 3-7 days in the cycle, but when a proper SW cycle takes a month or more that's not much.
The short answer is no, your not going to gain anything by doing it.
NO very dangerous to do especially since the bacteria in FW is alot different from saltwater plus you don't know what disease's could be in the water that don't bother FW but might kill SW fish