How do you plan on capturing the FM signal without a tuner of some sort (USB or PCI)? Its impossible.
The only way to do it with software only is to connect to an FM stations web stream and then rebroadcast it out with Nicecast or something similar, which would make no sense since you could just connect to the original stream.
So you need to capture the signal and then broadcast. Hence the USB capture dealy and Nicecast.