No, it's not. If a power supply is capable of a specific amperage or voltage on a given cable, it will already have it. Six pin cables are only rated for 75w while 8 pin cables, or cables with 6+2 pin connectors, are rated for 150w. In some cases you can use a 2 x6 pin to 8 pin adapter, but it's HIGHLY not recommended to adapt ANY power supply cable to something other than what it already is.
What is the EXACT model number of your power supply?