Propellor is free software, released under a BSD license. Anyone is free to modify and distribute it; indeed doing so is a fundamental part of how propellor is configured and used.
You don't have to contribute your patches, but you're encouraged to do so; let's all make propellor awesome together! Also, contributing patches is a good way to make sure that your code keeps working in new versions of propellor. See interface stability.
To certify that a change is released under propellor's BSD license,
you can use
git commit --signoff.
Propellor has its own coding style, which you are encouraged to follow -- at least when you want to get a patch merged into propellor. When writing code for your own internal use, feel free to ignore all the rules, except the ones about avoiding partial functions.
Also, mainline Propellor aims to be buildable with the version of ghc shipped in Debian stable. And, additional weighty dependencies are best avoided.