There is two ways to doing this.
Move the CF card to another system. Make sure to configure the correct drivers for the interfaces and setup the LAN IP.
Use a FreeBSD or pfSense machine, VMware works great for this, and mount the pfSense embedded image file. Change the configuration and then write it to the flash card. This is the method I do when I upgrade my pfSense box so I don't have to manually configure anything.
If interested in #2 you can check out the sh-mount-image.sh and sh-replace-config.sh scripts at http://wgnrs.dynalias.com/pfsense/
As far as the RJ45 to serial adapter would a Cisco console cable work? I know they are RJ45 to serial, just not sure if the pin out is the same. If so you should be able to pick one up for cheap.