Editing the iptables file on Asus firmware

I have downloaded the Asus DSL-n14u firmware, extracted the .trx file and located the iptables file, as linked below: https://www.dropbox.com/s/sanz5x2bw0o5xsu/iptables?dl=0 I know the iptables rules that I need to add. However, I am just a little confused about two things: 1) Is it possible to edit this file, add the rules, repackage the .trx file and then reload the firmware on the router? 2) If #1 is possible, where in the file would I add my custom rules? Thanks, just need a little guidance :)