you may have to buy a slightly bigger hat- but that risks it being too lose. some hats are quite different in shape- i tried on tonnes the other day as needed a new one- some seemed to have much more space around the ear area than others (i don't have an implant- just a random observation!). you don't want to compromise the hat- but could a bit of padding be removed?
hope you find a solution
I have no personal experience of this but would suggest that you contact Pro tector as I know they do made to measure inserts so may be able to help you.
I know someone who does it, but she has had a specialist helmet made. I don't know the details of this, or how expensive it would be. Otherwise, to ride in a well fitting helmet, she has to take them off/out. If she were to ride in a helmet that allowed her to wear the implants it would be dangerously too big.
Sorry I can't offer more help, but specialist helmets are available, which AFAIK are the only really safe option.