You can, I think Radio shack would have a splitter.
You do have to watch out for impedance (ohms) and overloading your Macs impedance for it's sound output. The more speakers you run on one output (say the left channel) the more the impedance, the more power you will need to drive all speakers and your ohms factor will double.
Example...4 ohm output, two 4 ohm speakers=8 ohms
You will need twice the amount of wattage to run those speakers and will put a load on your sound source because it is rated at a lower rating.