Why can't you add additional exercises?

I'm restarting stage one. I had a setback but now I'm back. I was wondering even though the book does not suggest spot training, why can I not add an exercise or two that I feel works. For instance, the back extensions that you do on that one thing where you are leaned over seem to really work fast for me in the past. I have some leftover pregnancy fat there that I would like to remove quickly. Also I used to do "good mornings" before getting pregnant to really define my butt and I noticed good results there. So my question is why can't I add those to my routine since the routines for me go by pretty fast? Just asking. :/

What's your diet like? Losing the extra fat will be more about your diet (i know i know i hate hearing that too, just irks me when other people say it). No matter what exercises you do, your body will have its own ideas of where it wants to lose fat first. So if you are really worried, the first thing I would suggest is to use that extra time that you are not doing 'good mornings' to do some healthy food prepping.

Personally for me the deadlifts, lunges step ups and squats did wonders for my butt.  Its never been so "perky" still big due to my diet but its got some good shape and firmness.

The good mornings / back extensions are done in phase 3 and 5 I think if I remember right -- or maybe 2 and 4  so be patient you'll get to them soon enough. 

And as the octo said what you put in your body food wise is going to be the biggest help for the body fat.

Well that is good to know. I just remember feeling the burn right in the center of my buttocks when doing those exercises.