Assuming you have two nics in the SBS box,
Make sure ports are open on the external firewall and being forwarded to the SBS external Ip address.
On the SBS box, open the ISA gui, run the publish a server wizard select the ftp server protocol, provide the internal IP address of the server that will be running the ftp service and select the external interface.
You will need additional IP addresses for each ftp server published and each will have its own publishing rule unless you want to vary port numbers which can become a bit convuluted.