i want to use a risers with 6pin cable but with molex or sata it's less power than the pcie needs(75)w and i don't want to use the main pcie entrances on the power supply becuse i want tp use them for the cards so i was thinking is there a cable i can use to convert this output to 8pin ? even if i use 2 to get one 8pin ? can i use that?
No. And its not recommended - they arent meant to handle that kind of amps and will likely burn/melt. If using a low power GPU use a molex riser. Or if you still want to use a pcie powered riser, get a splitter cable and use that on your psu's pcie cable - use one end of the splitter to power the riser and the other to power the GPU.