I'm getting rusty (the pun was truly unintentional) on this, but I believe there was an ipchains kernel module that loaded on top of an iptables architecture kernel. You could do most ipchains tasks with it. You could not mix rules (some ipchains rules and some iptables rules); it was an all-or-nothing switch.
I wrote userspace converters that respectively turned an ipfwadm firewall file into an ipchains firewall (ipfwadm2ipchains) or turned an ipchains firewall into an iptables firewall (ipchains2iptables). Neither created a perfect conversion (there are architectural differences in the firewalls that can't be perfectly converted), but both covered the majority of rules correctly.
The tools can be found at www.stearns.org/i2i/ . Like sausage, they're functional but ugly in their implementation.
-- Bill Stearns