Thank you for taking the time to leave your question! We can certainly look deeper into the PCIe power to SATA / Molex power adapters with you.
We do have adapters to connect SATA or Molex(LP4) power adapters to an 8-pin or 6-pin PCIe-powered device.
StarTech SATA to 6 Pin PCIe Adapter
Product ID: SATPCIEXADAP
Adapting power from the PCIe power to SATA, in the opposite direction, for use with devices requiring SATA power is another matter.
On the SATPCIEXADAP adapter, each SATA connection has a black and yellow cable. A SATA splitter like Product ID: PYO2SATA, there are black, yellow, orange, and red connections.
6in SATA Power Y Splitter Cable Adapter - M/F
Product ID: PYO2SATA
For clarity, we can compare the pinouts of these two cables. Here are a couple of pinout tables that can be found on Wikipedia
PCI Express - Wikipedia
Serial ATA - Wikipedia
The PCIe power connections have ground and 12V connections corresponding with the black and yellow cables.
The SATA power connector has ground, 12V, 5V, and 3.3V which correspond with black, yellow, red, and orange.
While we can certainly use the 12V power from the SATA power connection to power a device requiring 6-pin or 8-pin PCIe power, there are no 5V or 3.3V on the PCIe power connector for connecting devices requiring SATA power.
To add additional SATA power devices to your system, consider using a SATA splitter like the PYO2SATA posted above. Most SATA power connections can safely be split once, depending on the power needs of your equipment.
Let us know if you have questions!
Never hesitate to reach out to our Technical Advisory Team directly if you have any specific questions about StarTech.com products, or would assisting setting up a StarTech.com product.
David F - Senior Technical Advisor