Adding an Inquisitorial symbol is definitely a must, but I am not sure if some Grey Knights kit might have a suitable bit for it without looking enormous on him. I have inspected the sprue and I see that the hat is separate from the head. If you have it unassembled, I would suggest leaving the hat out and finding an alternative to replace it, as the commissar's hat is one of the most recognizable symbols; without it, people will not automatically assume he is a commissar. If you still want him to have a hat, I would suggest going for something a bit fancier (see Greyfax or some of the older inquisitor minis). Making a quick browse, I see that the main character of the Inquisitor: Martyr videogame has an iron halo over its head as part of his armour, that looks suitably pompous for an inqusitor so you could try something similar. After all, they are some of the most powerful individuals of the Imperium, so they can dress as fancy/extravagant as they feel like.