Well technically you can't have more than 100%. EDIT: That is without comparing an object to another object, or to the same object over a period of time, but tests are not done by comparison.
But you can indeed have 100% of the required amount and also have X% of a bonus amount.
For example:
19/20 (95%) on the test and then also
10/10 (100%) on the bonus questions of the test
But the problem with that is then the extra credit has to be weighted all funny like to actually mimic 10 points out of 0.
So the simplest way to add extra credit happens to be having it actually be extra credit.