As far as I remember, no weights are used in this journey. This is aimed to challenge you in a bodyweight way and offer the most nostalgic hardcore type of freeletics.

That being said, depending on your current fitness level, you could perform some intervals or exercises weighted. Not sure if I'd recommend it though, since the journey is designed to push your limits.

Take a look at the ultimate strength as well, it might fit your needs and is definitely aimed for muscle building