topower 500w

I'm about to buy a RX 570 (4gb version), the RX 570 has a 8-pin connector, but my Power Supply has just 2 6-pin cables (my power supply is 500W). The question is... It's safe using a x2 Molex to 8-pin (or a 6-pin to 8pin) adaptator? I can't spend on a new power supply (Here in Argentina it's...