I think you are pretty much out of luck unless you really gut the system, AND you are lucky.
Looking at the z623 design, it looks like the controls are in 1 of the satellite speaker cabinets, so you will need to go inside the cabinet to access the speaker wires.
This is assuming that the crossovers and amplifiers are in the subwoofer enclosure. And, that the other satellite is just a speaker.
I wouldn't try cutting into the cables since you don't know which wire is which, and because the cables will be made of the thinnest most fragile copper wires imaginable.
The acoustic design, crossover, driver design, etc etc are all contributing to the overall sound. There is little chance you could swap drivers (the speaker itself) and achieve the same sound as the creative. The creative speakers are likely full range speakers, and will sound completely different when driven by the logitech satellite amps.
You can try opening the satellite cabinets, disconnecting the wires from the speakers, and running wire to the speakers in the creative cabinets. But there are all sorts of things that might be different, like speaker impedance, etc.
You can also try opening the subwoofer (usually very difficult) and grabbing the speaker amp outputs and running wires to the creative speakers. But you still need the logitech satellite as a volume/etc control.
Probably the best you will get is a splitter to drive both systems, and go into the logitech and disconnect the speakers.
If the logitech had a puck for volume control (rather than in 1 of the satellites), you could just disconnect the satellites and only run the subwoofer.