It is not possible to directly use a PCIe cable that was included with your PSU but if you are handy with using a soldering iron or making custom pin crimpings, you can utilize this often unused connector on your PSU. Just be aware that the 4+4pin CPU connector has 4 +12V wires and 4 grounds, whereas the PCIe 8-pin has only 3 +12V and 5 grounds (one is used as a sense pin).

There are many pin-out diagrams on the Internet of the two connectors and how the wires are terminated, even the link bathrobehero posted has a good diagram with an overview of the differences. Always double-check with a volt meter before plugging any modified cables into your equipment to be sure you got the wiring right and you should be fine.