I'd say it is not easy doable, if doable at all. The much easier way to add extra juice to your PC is to put two PSU's in the box (if you can find room in there) and power up different devices with different PSU's...
The easiest way is to
- connect grounds from both PSU's together
- mod the power switch to power on the second psu at the same time it powers the first one
- do not connect + or - leads from both psu's together
- power the MB/GPU/SoundCard from one PSU
- power the HDD's, DVD's and fan's from second PSU
That will do the trick (do not forget to connect ground from one PSU to another, black wire) in most cases. You will get extra juice as you will split the load and you will not have to meddle with the insides of the PSU. I have used this for over a year (one box with 2 atx psu's and one box with 1 atx and 1 at psu) without a problem, but, I do not guarantee that this solution is problem free, maybe someone more experienced can comment on this proposal
As for the "meddling" with the PSU insides, note that most PSU's if not all have few built in protections (short circuit, overload...) so you would have to modify those circuits too in order to allow PSU to put out more juice... not for noob's and easy way to release the magic smoke from your PC