I'm not a modding expert but I can imagine that most modern PC games can be modded in one way or another, whether the developers support it or not. The folks behind Total War: Rome II fall into the former group as they have just released the Beta version of the Total War: Rome II Assembly Kit. The kit is free to download and comes with a suite of mod tools designed to help modders edit the game's database and variants as well as process data such as textures, animations, campaign start positions, and more. What's more is that once your mod is complete you can upload it directly to the Steam Workshop via the ROME II launcher. More info can be found over at the Total War: Rome II Wiki, so head on over and see what you can do.