I think it depends what "phase" you're in.
I put that in quotes because it's something I'm defining myself, not something you'll find in the book.
If you're an active person who has trained in other physical activities before, and you're just starting out with these as new skills, then yes you can easily do other things while you train and learn S&S. The weight you're using is not stressful for your body. You just need some practice time before moving up. (sounds like where you are,
@arielbe)
If you're past the beginning phase and you know the movements and are starting to use a challenging weight, then no you don't want to be adding stuff. (you will be there soon,
@arielbe)
If you're really into the meat of the program and doing goblet squats with heavier weight, really powerful and challenging 10x10 swings, and get-ups that are very difficult, then no you don't want to be adding stuff. Let this stimulus drive the adaptations that add strength, muscle, and capability. Give yourself quality recovery. Follow the book/program. (you will be here also, if you choose to,
@arielbe. The general recommendation is to be here until reaching the Simple standard, or at least Timeless Simple).
If you've achieved Simple or some other milestone and you are using S&S for maintenance (not really trying to push into any heavier weights), then yes you can easily do other stuff. You have adapted to the stimulus and can use other things to drive new adaptations. S&S becomes like your daily walk. Activity, but not your training program. (sounds like where you are
@offwidth)
If you are used to a lot of training volume and are good at juggling programs, and are also doing a serious training program, or some sort of martial arts, or endurance training, or something else, then S&S 2-3x/week can be helpful to take a milder dose of stimulus. You will progress, but not as fast. The ability to recover is key, as
@Bauer said.
S&S is a pretty flexible program, actually. It just depends if you want it to be your focus. The more you focus on it, the more you'll get out of it. But it can still offer benefits when it's not the primary focus.