I realized that weak abdominals are resulting in poor Squat form and thus limiting my progress in StrongLifts 5x5. How can I integrate and program additional abdominal workouts to my StrongLifts program?
Don't. The issue is not "weak abs" but that you haven't been properly engaging your abs during squats. Heavy squats, unlike light squats, require a locked trunk, which requires abdominal engagement. This doesn't mean you need to do ab-specific work. It means you need to set up your squats correctly.
Additional ab work can be OK, or it can distract. I suspect that in your situation it will distract. Stick to a program, don't veer off into assistance exercises, and focus on getting strong in the core lifts: squat, deadlift, overhead press, two or so more.
I'm going to differ from Dave's answer and throw out some ab exercises, because I feel that done right they'll accomplish two things:
- Your abs will get stronger, which is important but not so much as the next item.
- You're gaining motor unit recruitment, and learning specifically how to engage them.
A good example of this for me (and others I've seen) is the ab wheel rollout, and it's an exercise I'd recommend you start incorporating. Done incorrectly, people feel it in their back, which is bad. To do it right, you need to isometrically lock your abdominal muscles. So there's value in the exercise from a strength standpoint, but there's also value in that you really learn to control and lock your abdominal wall.
I think something like a barbell glute bridge is likewise good for deadlifts in that beyond the strength gains you're practicing the engagement of a particular movement which is helpful to many others (kb swings, deadlifts, back squats, etc).
The other "abdominal" exercise I'd recommend is the front squat. Not for any weights that would interfere with your 5x5 program, but enough that you are again having to extend and brace (isometrically contract) your abs from buckling.
When doing an overhead press, I think of the lock out on the top of a deadlift to remember how tight my glutes should be. I don't think there's anything wrong with doing some ancillary exercises when they're not interfering with your primary training program and you're doing them to work on an issue that you've identified.
I get Dave's sentiment though, and I say it often enough myself, which is basically "Don't get creative: stick to the program, you're not smarter than the person who designed it." But you can mitigate that by keeping the ancillary exercises light and maybe once a week.