Well if it says "bw" that means you don't add weight. But 3x10 back extensions with no added weight seems silly.
I'd either add total bodyweight reps, until you reached around 100, or add weight. If you're using a 45 degree back extension, you can easily hold a barbell to add as much weight as you like. Otherwise you have to settle for a DB held across the chest.
But I don't like the idea of swapping out deadlifts for back extensions, even part of the time, barring deload, or backing off DLs to focus more on other lifts that tax the PC. If you're finding it difficult to deadlift every other workout, I'd suggest (A) have your technique checked, and (B) Do lighter deadlifts and high rep back extensions every other time you deadlift.