Add a Thick Grip
I've been slow to adopt the thick-grip training craze, especially with pulling movements like rows, chin-ups, and deadlifts. It sounds good in theory, but in practice using a thick grip forces you to reduce the weight so much that you end up sacrificing overall strength and muscle building potential for the sake of grip strength or forearm size. Not a worthwhile tradeoff.
But It's Great for Face-Pulls
Adding thick grips to face pulls is different, though. Remember, face-pulls are great for posture, overall shoulder health, rear delts, rhomboids, and external rotators. Most people should be doing them.
The Basic Face-Pull
Face pulls are more of a "feel" exercise than a brute strength exercise, so you don't want to go super heavy anyway. The weight you use won't be negatively affected by the addition of the thick handles.
Moreover, one problem with face pulls is getting too fast with the movement and using the arms too much, which defeats the intended purpose of working the upper back. Adding thick handles helps you take the arms out of it, allowing you to think about using your upper back to do the work. Not only do you get some extra work for the forearms, but you also make the exercise more effective for the upper back.
Just slide the thick handles onto a triceps pushdown rope and go. Just don't try to be a hero and use a ton of weight. No one gives a rat's ass how much you face pull.