That keypad does not have a comma.
You would need to press a comma on your main keyboard, when you need that.
There IS a utility that allows you to change what a key does. But, if you change the period to a comma, then you won't have a period when you need a period.
But, maybe you are OK with that. Karabiner can do that... (I don't know if that will work with your external keypad - you will need to try it for yourself)