Hello!
You don't need to create the mod. You could just extract the core.cpk file (and delete this file), edit a single file in script (see below). Then the game will run with the extracted core.cpk. That's something we've recently implemented to simplify the modding.
You need to edit just a one line of script located at:
(from core.cpk) data\scripts\internal\GenerateGalaxy.js
This line:
NumOfSystems = Math.floor(50 + 50 * (args.galaxy_size / 255));
Replace it with, for example:
NumOfSystems = 300;
Then when you start the new game on the server, it will generate a new galaxy with the specified number of the systems.
So, you don't need to create the mod. Backup core.cpk, extract it into the same folder where core.cpk was located and delete "core.cpk" (so there will be folder "core" or "core.cpk" instead of archive "core"). The game will use the files from the extracted folder.
Regards!