Yes, you certainly can! You can add sessions from another program on the days when you're not training your lower body with The Glute Project. Be sure to check out the resource section in The Glute Project, as it includes an ebook that explains how to structure your training program across two programs and provides suggestions on how your schedule could look.
To switch between programs, simply select the switch icon 🔀 below your name in the profile section of the app. Each time you switch between programs, make sure to select "Continue where you left off" so you don't lose any progress in your programs. This will help you stay on track and keep your progress intact as you move between different sessions!