There are a ton of bindings and customizations in the way things start up, the way things are configured in FreeBSD, etc. You'd need to either use our whole build system (which is a real pain), or write your own init and make a lot of customizations to the pfSense code. Might be better to start from m0n0wall depending on what you want to do. Or if it's something related to pfSense, it would likely make the most sense to just create a package and avoid the pain and many, many hours that go into creating your own system like this from scratch.