You would first need to know some more specific information about the amp you intend to modify. Specifically, the circuit configuration and the rail voltages and the current demands need to be understood. Then the power supply would have to be designed to meet those same requirements. Here's a pretty good tutorial with some insight into power supply design for audio power amps. To actually modify the car amp the DC-DC converter would have to be disconnected or removed and replaced with the newly built supply.