Connection Tracking module for ip_nat_netbios
Connection tracking is implemented for typical tcp-protocols like FTP. Microsoft netbios based protocolls needs conntrack mode also.
A sample solution is available e.g. from http://www.cse.unsw.edu.au/~matthewc/files/ip_nat_netbios.c, but other solutions should be available.
For Firewalls between company networks with WINS and netbios protocol and NAT at the same time a connection tracking would be needed.
A VPN is not an option since the company networks need NAT regardless of use of an VPN or not.
Mario Schmidt commented
You better use a VPN instead.