That is not easy.
But it is possible. The
pcap library, used by
Wireshark, or more specifically its
WinPcap implementation, can do just that. You'll obviously need to be administrator (or root), but it can then grab any packet that enters or leaves your computer, and eventually create your own. I used it to "power-up" the broadcasting from games like warcraft III to a list of reachable known IPs outside of broadcast range.
But it is
completely outside of SFML scope.