I thought of this quote from madcow:
"
Substituting Exercises:
Don't fuck with this. Every BBer seems to have Attention Deficit Disorder and an overwhelming desire to customize everything. The bottom line is that these are all the most effective exercises and just about anything one does will result in less gains. As a rule those people who want to change it don't know enough to make proper alterations - those who do know enough, don't have much to change. The two guys who are responsible for this program are some of the best on the planet at bulking lifters and making people stronger."
Then realised you're not substituting exercises. But you are adding extra work, which means extra recovery is required. And you're not getting extra recovery, you're getting less as you're adding that work on Saturday - during the only two day rest period you have, and prior to Monday, which is the PR attempt day.
You won't know unless you try it, but personally I wouldn't. If you can get away with it and keep making PRs on the Monday, with extra arm development, then that's great, but I suspect adding work and reducing recovery will affect the PRs.
You might want to start tracking weight instead to make sure you're eating enough. I know my arms have gone up from 13" to 13.5" over the past month or so (and my gut's gone up from 33.75" to 35" but that's another matter). My weight's up to 74kg, about 4kg in as many weeks.
I should be the one complaining about upper body strength anyway - you're stronger then me on all the upper body lifts, but I beat you on lower body.
