Oooor just drop them on a USB stick and plug it in.
Get a high-capacity USB stick and throw all of your music on it. Right now I have a 32GB Corsair Survivor USB3.0 stick that works wonderfully but is about 300MB from full capacity. About to buy the 64GB version and let it fly. It's also borderline indestructible, waterproof, and looks sweet. I bring it inside, drop any new music on it in 5 sec, and then plug it back into the car. Normally it stays plugged in and I have all of my music with me at all times.