The best option should be collecting whole new speakers in pairs of 2. Integrated speaker system where all amplifiers are in one speaker are very hard to expand. They are one package, and that is it. They can be deasent, like logiteck speakers normally are.
I am not sure if it can be done, but you could try to buy active speakers that each have it's own amplifier and put the volume down from the originals. The problem is that you have to have sound system that have individual output for each pair of speakers...
Other is that you buy external amplifier and normal hifi-speakers, but how do you get sound from there to your Logitec system... hmmm... maybe using rca out connector. Most propably the sound balance is somewhat weird... But then you have an option to buy later 2 speakers more, then one more and the sub...