Do the SATA power cables come directly from the power supply, or do they connect to a regular four-pin (Molex) plug that leads to the power supply? If the latter, seems you can insert a "Y" splitter cable into that chain. If the former, well you could splice something - see attached for ideas. It does seem odd that there isn't another power plug somewhere, what was supposed to happen if you wanted to add a second graphics card or another hard drive, is this thing not expandable?
SATA Power Adapters
It would be ideal to figure out why the mobo is turning down the fan speed despite the temp sensor showing hot. But sometimes "ideal" isn't worth it.