If you mean to continually redefine the 25 available Note Flag slots,
then yes, you could have more than 25. But if you're assigning meaning
to the flags, this isn't always an option.
I think the original poster was asking if you could add to the palette of
predefined symbols that one can use to customize the 25 Note Flag slots,
and the answer is No. For example, I can't import a BMP of a bunny with
pancakes on its head or a donkey on a waffle and turn it into a Flag.