Since we have no way to control the volume level of the door chimes out of the factory system, the only answer is to adjust the gain staging between the converter and the aftermarket amp to minimize how loud the door chimes are.
A good way to do this is to cut back on the volume output of the converter, and increase the gain input on the amp accordingly.
Since the door chime is at a set level vs. the variable level output of the head unit, by decreasing the over all gain output of the converter will make the overall level of the door chime lower going to the aftermarket amp.