I recently bought a pretty hefty PSU (600W) that literally doesn't have enough power cords sticking out of it to satisfy all my needs. I don't even have my DVD drive hooked up because I can't power it (which is okay since there's very little use for it anyway). The two RAID 0 HDD pairs are largely to blame here. The bigger problem is that I will be receiving yet another hard drive that I need to power soon and I'm not really bent on buying a new PSU just because of that. Are there any easier solutions (can I reliably split a power line coming out of the PSU? Are there any decent external solutions anyone can recommend?) that would avoid me the hassle?

600W is not much for a modern PC, a single GPU card can easily use 250/300W.

What you can do is first checking if your PSU feeds enough current for your current hardware and if you can have enough spare power to add another one. If so, check that you do not reach max power for a single line, if it still good you can safely split that line with the correct adaptator.