I wouldn't look to convert/adapt any other power source, no.
If you want to use the 8+4EPS, then you'll need a PSU with native support for that.
FWIW, depending on the specific CX750M (Green, or Grey label?), it may not be a great offering for your setup anyway.
Typically, you're looking at a higher 'tier' PSU for >8pin EPS.
Those would be higher wattage units (750W+, generally) and from top-tier manufacturers like SeaSonic, SuperFlower etc - OR, from the EVGA/Corsair's of the world, on units manufactured by SeaSonic et al.
Focus+ Gold, RM750x, SuperNova G3 750W (or higher wattage units) would all be examples.