Hello Nick
In the FPP you are doing 5 sets of Pullups 5 days a week which is plenty - any more direct Pullup work is probably overkill!
Aside from that I've heard Pavel recommend doing Hanging Leg Raises for more abdominal Strength in the Pullup as well as working on the Hollow Body Position (google Hollow Body Rocks for a basic exercise to improve the Hollow Position). Also doing some Neutral Grip Pullups here and there might be beneficial because for most people it is the grip variation that is the least likely to produce shoulder pain. This means that you might be able to go harder/heavier on the Pullups without tweaking something.
Don't write Weighted Pullups off neither, because they are in my opinion the best way to work both on Pullup Strength and Endurance. The higher the added weight, the more numbers you will be able to crunch out with bodyweight only. All that is needed is a Dip Belt or a sturdy backpack to put the plates in. Or hang a Kettlebell on your foot...
The FPP can also be done weighted to attack your 3RM or 5RM.
Hope that helps!
