i was wrong, it is modable, you got to modify the Entity.manifest in the entrenchment folder to add new entities to entrenchment (anything not already in the entrenchment gameinfo, this includes if you want to modify any original sins research, you got to add it to entrenchment folder and then add it to the entity.manifest).
the mod folder should be something like Mods/(modname)/Entrenchment/GameInfo + Entity.manufest + any other modifyed files. of course i havent tested this, and im not 100% sure if the new mod loader works correctly. i hard moded all my files directly into the install directory since with every new beta you got to reinstall anyhow.