If it uses a banana plug connection, you can buy Y splitters for it, same if it uses RCA cables.
If it doesn't have any special connector (just a stripped wire in a spring terminal or wrapped around a screw post) you can connect the wires with a wire nut.
Ah, then you won't need any special adapters for it.
I did this with my stereo, and it works fine (I have 8 speakers hooked up to a tuner that should only have two xD). It's not the best way, but it's the cheapest, and it works.
What you need to do is take the negative wires from the left speakers and splice them together with a short (maybe six inches) length of speaker wire, then do the same with the positive cables.
Do the same thing with the negative and positive cables from the right speakers. Now you can just insert the short wires in the plugs on your stereo.
You should use wing nuts for this (they're very cheap) but you can get away with just twisting them and wrapping electrical tape around the exposed wires so the negative and positive don't touch.