Your iMac has built-in optical audio out, which means the only way you're going to "hear" the real DD 5.1 or DTS out of it is to connect to something that supports it, i.e. something that has optical in. That something has to be able to decode the digital information, convert it to analog, route it to your speakers and drive those speakers.
What I want to do is to hear 5.1 dolby digital surround sound from the creative speaker (which does not support DD and DTS like Logitech z5500 and Logitech Z906 do)
If it doesn't support DD and DTS 5.1 inherently, then you're not going to hear it. At best you'll hear "pretend" surround, i.e. the Inspire will make sure "sound" is coming from all your speakers but it won't be correct or nuanced.
1. Buy a Griffin Firewave...
Piece of junk, don't bother. It didn't actually support true DD 5.1, it merely simulated surround sound and did that badly. Way back when, circa the G4 mac mini era, the only way to get true DD 5.1 passed to a device over firewire was with an M-Audio interface, and then only if you managed to get their drivers successfully installed on your Mac. (Which was no guarantee since M-Audio was very, very slow in adapting to new OS updates from Apple.) The Firewire Solo is what I used with my G4 Mini and a PowerMac, but I was so glad when Apple started shipping Macs with optical digital audio out. M-Audio was the only company that tried to enable this in OS X, since Apple didn't include this functionality inherently in its native firewire protocols.
Your goal in all of this should be "passthrough," to get the Mac to send the DD 5.1/DTS untouched to a device (like a real AVR) that's capable of decoding that digital audio information, processing it and then routing it to your speakers. The problem you have is that no matter what you do, the "processor" inside the Creative Inspire system, inside the sub which routes the signal to and drives the speakers, doesn't know what to do with digital audio streams...so whatever intermediate device you try to stick in between your Mac and the Creative Inpire sub input I fear will still only be able to send audio to the Inspire over a crappy 2 channel analog connection because that's the only kind of input it understands.
Buy a external sound card like this:
Don't bother, Apple doesn't support passing encoded digital audio output like DD 5.1 or DTS over USB anyway, so external USB DACs and devices like this Diamond Xtreme are a waste of time for movie audio. Your Mac will only send stereo PCM over USB, which means it doesn't matter what you have connected to the other end, that's all it will receive (true surround sound cannot magically be recreated out of a 2 channel stereo input, at best you'd get a low quality guess. Here again, there were M-Audio USB devices that did correctly pass DD 5.1 through, but they won't help you either.)
Well, for that to be true, you'd have to find a way to connect each of your Creative speakers to one of these decoders/converters, and bypass the built-in processor of the Creative Inspire, which probably isn't possible, but let's say it is...then, you'd have to trust these manufacturers that they have a real DAC chip inside their device that's capable of receiving and properly decoding DD 5.1 and DTS, the same kind of a DAC that's inside your typical AVR. Then what you hear might actually be the digital audio mix that was included on the original movie. Now, this last bit, that a device like this may have a perfectly adequate DAC chip inside it that's capable of decoding DD 5.1 and DTS just like a real AVR is possible--DAC chips were commoditized a very long time ago and are very, very inexpensive. And the inexpensive ones do such a good job that they are usually indistinguishable from extremely expensive gear! So it is possible converter/decoders like the ones you linked to could work, i.e. could pass the center-channel audio to the center front speaker as the director and sound engineer for the discs and movies intended. The problem I suspect you'll run into is your Inspire speakers don't exist on their own, they are not separate active or passive speakers, they can only work when all hooked up together as a unit through the sub, fed by the sub. If that's the case, then you're back to where you started, the Inspire will only see 2 channel PCM coming in because that's all it knows how to receive...and then it will create some sort of pretend surround sound on its own, again, because that's all it is capable of doing. Real AVRs are much, much better at this sort of thing, decoding digital audio directly from your Mac and managing different surround audio profiles.
What you want to do, is sell (or donate) your current speaker system and upgrade to a similar powered system that has an optical input, or much, much better, step up to big boy audio: do not spend any money on the Logitechs or any of these linked devices, instead buy your first real AVR and real speakers. Buy someone else's used equipment if you are on a tight budget, just make sure the AVR has optical inputs, something from 2004 or so should suit you just perfectly. Plenty of very good, very inexpensive equipment can be had via Craigslist as audiophiles and audiophools upgrade to the latest and greatest with HDMI inputs. Why not benefit from that? Once you have an AVR with an optical input then you can start gradually adding speakers as your budget allows.