I am not quite sure what you mean. (I get confused with all the different NATs) I think PAT is just going from many to one, isn't it?
You can match a packet on a given interface (the external one) for destination, source, and port, and then change its destination, source, and port to assign it somewhere inside your network.
Let me know if that isn't what you meant, but the docs are excellent if you get stuck.
In packet filter/rules do u have from (client):any sevice[:$]ddport222 to (server):internal server action:allow
Guessing it's ftp you want in due to port 21 on the external interface Have you defined your server in Definitions/network ? Name:your server IP:your servers ip subnet mask:255.255.255.255