If the existing filesystem can be read by FreeBSD (like ext), you can just copy the data over in a single system but in the worst case, you could just copy the data over from a 2nd system over the network. You'll probably want to do that using tar with archive options, so the only difference between doing it locally vs over the network is whether you pipe the tar create directly back to tar extract, or to a tcp utility like tcputils or netcat in between.