FTP USER+IP ASSOCIATION CONTROL

[Comunicaciones Servinform]

We need have our linux iptables firewall making an ftp user+ip association control in external public interface. 
If ftp user+ip association is correct we need transfer ftp ssl connection to internal Ipswitch WS_FTP Server 5 with user db in sql server database. 

Firewall: 
linux suse 8.1 kernel 2.4
iptables v1.2.7a

Internal server:
w2000 server
sql server 2000
ipswitch ws_ftp server 5.0 with forced ssl connections

It's possible? Any idea?. Thanks in advanced