What do you mean? You mean get a car receiver and amp and hook it up to your existing speakers..? It would be easier to just get a Home Theater Receiver since they can put out upwards of 80W a channel, compared to a car receiver which puts out like 15W, plus, I wouldn't even know how to hook that up.
Since a car audio system is meant to operate on 12volts dc you would need to buy a large 12v dc power supply to get it working (or use a car battery and charge it when it goes flat). Probably better and cheaper to buy a cheap stereo receiver or something used. Car stereo power ratings are pretty inaccurate so don't assume that the power will be adequate.