Ecto's generally respond better to low volume. For now I would add probably just 1 exercise for 2 sets doing 6-8 reps. PRCal mentions working in the 12-15 rep range, which is fine for a more advanced trainee but I think your way to weak to bother with hyperthropy work at this point.
So at the end of a workout A, do something like 2 sets of standing barbell curls for 6-8 reps. And with workout B do another exercise for whatever body part you want to work with the same rep range.
Remember that more is not always better.
I recently changed my routine to one very similiar to SS, 2 compound exercises using 5x5 and 2 assistance exercises which is way lower volume then I had been working with for a long time. Without any direct arm work, my arms have grown 0.5 inch basically overnight and I have been stuck at 17inches for about 4 months, even after gaining ~20lbs and having a day dedicated to arms alone.
Also, don't worry so much about upper chest, lower chest, outer chest. Along with most people, I found that it all ends up delveloping fairly evenly when you start gaining some overall mass.
Remember Rippetoe has been training people for 30 years. If he says not to do something, there is generally a reason why.