Sure, just have to modify the image. Any changes you make at run time will be lost at the next reboot (though that would be a good way to first test your changes, if you really blow up something, just reboot
)
Install a FreeBSD 6.2 machine or VM, and grab workon.sh from
http://chrisbuechler.com/m0n0wall/downloads/run:
chmod +x workon.sh
./workon.sh imagename.img
then 'ls', and you'll see the mounted directories. Make your changes, type 'exit', and your image will be re-packed with your changes. You can then load your newly modified image using firmware upgrade.