Absolutely, positively do NOT do that! A molex to PCIe adapter is bad enough, but then trying to trick the video-card-side adapter into thinking two molexes are providing power when only one really is, well, that's asking for a fire.
That they don't mention what the PSU is, on its own, is a little alarming. That it's a modern system, but the PSU has 4-pin molex connectors and does NOT have PCIe connectors is even more alarming, making me wonder if the PSU is one of VERY old design.
If
this is showing the correct specs, then, for all intents and purposes really a 408W PSU. Thus, even if it had the correct PCIe connectors, I would NOT trust it.
Take a look at the first link in my signature for PSU recommendations. Get a high quality unit, and enjoy your PC with the new video card trouble-free and hazard-free.