Yes, but it's not trivial. You'd have to block the connection from the power supply for each voltage (BTW - it's 5 volts, not 5.5 - if yours is 5.5, it's too high), probably by putting tape on the contact for that voltage, then you'd use a multimeter with a high current ammeter to bring that voltage to the motherboard. A 2 amp ammeter should be fine for a 6500, but on startup the initial current would probably exceed 2 amps for a few seconds (which most ammeters can handle just fine).
A 6500 in general shouldn't take much power. The thing to watch for is the difference between continuous rated power and peak power. Some cheap power supplies only tell you peak, but good ones (with active PFC, Plus-80 type) will tell you the continuous rating. A 100 watt continuous would be more than enough to run a 6500.