The PSU-end of the modular connections has NO widely-used standards. Various brands from the same company will often differ in pinouts.
The 6-pin connection on the end of the PSU cannot be assumed to be equivalent to a 6-pin PCIe connector (and almost definitely isn't).
You COULD look up a pinout diagram for your specific unit (or make one with a multimeter), then fashion your own cable together (keep in mind that you can't expect to draw as much current, because the "peripheral" connections will likely not have as many 12V lines as the dedicated 6-pin connections, since PCIe power doesn't require 5V that is required for molex, or the 3.3V line that is sometimes used for SATA [it's sometimes excluded, as it's almost never used]). Otherwise, stick with using the modular lines of 4-pin molex power connectors as-intended, and use 2x molex to 1x 6-pin adapters (don't power too many risers off of the same line of connectors, obviously.)
