If it involves code at all, then it's not a mod, it's a change to the game itself and must be done by the developers.
Modding can just change the data values that aren't hard coded. For example, there are data files for all the ships and the tech advancements. So, you can create your own ships by just adding/editing a text file. And, you can add extra research by editing a text file.
But unless there was a file somewhere that listed the values for other stuff you can create, you won't be able to add a text entry for stars.
At least, that is my understanding so far.