Can I "trick" a PC Powersupply to power something other than a Motherboard?
Sure thing!
Those connectors you're describing seem to be 6 pin PCI Express power connectors, where top width of the pins will be 12 V (usually yellow cable) and the others (bottom row) connected to ground. These are usually shared across rails inside the PSU (usually different physical switching semiconductors, to spread the load).

In order to fire up an ATX power supply, you'll need to pull the power-on signal low by connecting it to ground. This is pin 16 (a green wire) on the main 20/24 pin connector. Just short that to any ground (black) wire on the 20/24 pin main connector.

Whilst a little unsafe, it can be done.

Also, as the PSU uses a switch-mode configuration to regulate line voltage down to ATX voltages you need to consider the loading on the power supply. An unloaded switch mode power supply without any protection can oscillate in ways that were not intended for its usually operation. Some PSU manufacturers cut costs by omitting components that help stabilise the power supply with no/low loading and count in it being used to power a PC only.