If you modify engine it is ok to distribute it in github as long its fork of original github repo, so its guarded same way as orinal repo.
You are free to distribute your own modules (this including plugins), as long as it follow EULA engine usage rules and you not coping code from engine, which is copyrighted.
in techical issue, it depends on how extendable thing you trying to modify via and it very a lot in many features, it really depend on your case. se in automation tool have any ui extenders