There's a constructor marked "use an existing socket handle", which takes a socket_t. You can cast an int to socket_t (socket_t is an enum based on int on Unix systems).
// it requires an address family but i don't think it matters much
auto socket = new Socket(cast(socket_t) 0, AddressFamily.INET);
That will get you constructed, though then using it results in "Socket operation on non-socket" errors, because D's Socket calls send() and recv() rather that write() and read(), so I'm not sure how to actually make it useful...