It could work, if you know what you are doing. I've got the 6-pin from my PSU connected to my GTX 280, and I have 3 4-pin connectors, wired up to an 8-pin plug I pulled off my old, broken PSU, and it seems to work with no issues. Just make sure you know what you are wiring to what.
Yeah... that would certainly qualify under the possible, but extremely
not recommended category.