Try in g_main.c under src/game.
If you add a stage, keep in mind that the stages are zero based. In game, if you want humans to be stage 3 you set the stage to 2. I don't know why they did that unless they just got done working with arrays.
If you do make this, make a stage check method that checks the stage of all teams and sets it to the highest playable stage if it is over. My team got set to stage 4 on popupman's server, and no, it was not a glitch. He changed the build points of his team only to some outrageous number the previous match. A stage checker would prevent this kind of foul play.