I dont think that writing in ball point pen at most 4 digits in a corner would be considered an attempt to deface money to the point where it is no longer legal tender.
Regardless of that, noone is going to get caught for having a bill with writing on it. Even if you did, they can't prove you did it. I get money with marker and stuff on it all the time.
I'm not saying you should go write on money. Use your own judgement, not mine
COry