Multiple speaker setup

I have Sound Blaster Audigy Gamer and Logitech Z-560. I already have the Logitech's hooked up and work great. I got this new speaker set for free and was wondering if it was possible to set them up so they play at the same time, adding an extra 100 watts to have a mutiple speaker setup. How would I do this?
 
No, all the wires from my 4 speakers of the Z-560's are connected to the subwoofer. There are no open slots for anything
 
The only other way I can think of short of buying and amp is to split the wires from the subwoofer and add them to the new set. If you use an amp you can then run everything from the amp and connect the amp to your sound card.
 
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