I use CG as shorthand for Campaign Game, as opposed to battle scenarios. I use scenario(s) generally to refer to both.
I would start by reading this:
http://www.paradoxian.org/hoi2wiki/index.php/Mods
the modding guides could be informative. These were written for HOI2 not AOD, so they're not 100% accurate, but provide an overview of how things work.
The file you want change is an .INC file. The .INC files set the starting parameters for each country in all the scenarios.
To add-to, or change the brigades on Yamato & Musashi in the 38CG, the file you need to change is: scenarios/1938/jap.inc
New construction is at the bottom of the file. They are the entries called division_development.
Now open up file: scenarios/1945/us.inc. This is US1945CG .INC file. Down in the division_development section is an entry for the "USS Kentucky".
This creates a build for an Iowa class BB with 5 brigades attached. The "extras" are the brigades and the corresponding "brigade_model" sets the tech level.
Familiarize yourself with this, and if you're still interested I can walk you through the changes.