You can use portforwarding... If you use a linux box to do the nat service and the kernel is 2.2.x use the follow rule ...
first you must load de portfw module.. modprobe portfw if you ip address is like 200.203.195.1 and you local machine is 10.0.0.2 use this to forward ftp connections ipmasqadm portfw -a -P tcp -L 200.203.195.1 21 -R 10.0.0.2 21
