When you say "home speakers" I'm assuming you mean non-amplified speakers. To wire such home speakers to your iMac, you'll need:
1) A receiver or amp to power speakers
2) A Stereo RCA to stereo miniplug Y-connector
Wire the speakers to the receiver or amp, then use the Y-connector to connect the receiver to the iMac via its headphone output and you're done. ** Make sure the receiver is powered down before you start wiring speakers and plugging things into it **
Griffin Powermate? I think you mean Griffin Powerwave. Well, a Powerwave will also work but if your main goal is simply to just power home speakers, then you're better off with something with more power output than the 15W x 2 that the Powerwave has. For $99, that's not a lot of bang for the buck and if you happen to have decent speakers, 15 watts per channel is weak sauce if you want any decent sound and will not do nice speakers any bit of justice. Instead for $99, you can get a decent set of powered 2.1 speaker systems (with subwoofer) that will be ten times better than the output you get from the powerwave powering your home speakers. There are many nice ones like Monsoon, Altec Lansing, and Harman Kardon such as the Creatures which can be had for about $99 online.