You definitely CANNOT add an amplifier. You could replace it, however. BUT, if you're not happy with its current performance, its probably best to start over. Your current device was designed with certain goals in mind and the components were chosen accordingly. Its very possible that eq'ing the system differently and adding wattage might make it perform noticeably better, but certainly no guarantee. To achieve optimal performance you'd likely have to also change the enclosure. And then, perhaps the driver wouldn't meet the requirements.
It will be a much less painful process to determine your subwoofer needs and begin with a clean slate than to modify what you have.
Thats the generic answer, but perhaps someone with more specific knowledge of that sub and its design could better estimate the benefits of more power.