No, you can't use internal and external speakers simultaneously. However, you can use internal or external speakers simultaneously with remote speakers, using AirTunes. If you have an AirPort Express, just plug your external speakers into that and select "Multiple Speakers" from the iTunes speaker selection menu.
Using the method I described, you can get sound from all 4 speakers. However, it will not be a true surround sound setup, as the sound will still be only 2 channels.
That still makes no sense at all. More sound does not mean more quality.
As for using the iMac speakers as a center channel, I'd personally advice against it. The iMac's speakers are of such a horrible quality that you'd do more to ruin your soundstage than do good with it. Properly spaced and positioned, the two soundsticks should give you a good enough soundstage. If you're after louder sound, then I don't know why you bought multimedia speakers in the first place. You could have gotten a decent pair of canadian bookshelf speakers like psb or energy and t-amp for under 300usd.
That's why I said more speakers = more sound (not more quality). I'm not suggesting that it's the ideal solution or that it's an audiophile's dream, but it does make sense that someone would want sound coming from more speakers. I agree that the internal speakers aren't of high quality, but there are cases for using four speakers instead of two.