You can download the mod tools from here, these are the official tools from Relic, have a look through the forums and see if it is something that you would like to start doing.
LinkThe mod that I created for the Space Marines increases a lot of things, like Terminators are now 3 (you can have 3 squads of each Terminators), Gray Knights are now 2, you can now have 2 Orbital Relays also increase how many squads they can hold.
Population increased to 50, heaps of other stuff to go along with this, too much to remember...
If your interested, I will upload it to my website for you if you want to have a look, but it is only for the Space Marines, if you try to play online or even watch someone elses replay you will get a "data sync error" from the game.