What is being described is sometimes called a "counterstamp". There are other terms that describe the same thing, but this is the one I learned to use.
A coin or or token is stamped with an image after it leaves the mint. This image can be whatever someone wants. I remember seeing advertisements for halves that had a small image of Lincoln added that faced Kennedy. These were sold with a "Letter of Authenticity" that included some obscure "facts" about both and sell for many times the face value of the original coin.
I commonly see a few counterstamped coins every week. Letters, numbers, names, designs. You name it. Someone has probably stamped it on a coin. They are not rare by any means, but from time to time, I will see one that I like and keep it.
It could be fun to set these aside to see what people with too much time will do. Enjoy!