Look at these pages:
I've never had to use the raw table nor the NOTRACK target so my info is likely hardly authoritative. But, I guess it would work something like this:
$ipt -t raw -A PREROUTING -d -p tcp -m multiport --dports 20,21 -j NOTRACK
$ipt -t raw -A PREROUTING -s -p tcp -m multiport --dports 20,21 -j NOTRACK
If I understand things correctly, this should make connections to/from the specified FTP server untracked.
After that, you'd need static NAT rules to forward packets to and from the FTP server. I don't know if Netfilter does NAT without connection tracking. If not, then maybe iproute2 can help you there: http://linux-ip.net/html/nat-stateless.html