Can I add isolations, more exercises or other accessory work like abs?

If you feel like you can take it, you definitely can.

However, the flow elements can be incredibly taxing for the core, shoulders, arms and other body parts so it's not 100% necessary. 

You can add some of these workouts in the end of your main workout:

• Complete Core Workout for Six Pack Abs
• Mobility Work for Lower and Middle Traps
• How to Train All 3-Heads of Triceps

Other exercises include bicep curls, shoulder raises, leg exercises and so on.