I have a friend that has modified an AT power supply to be a power supply for lab equipment. He has a number of voltage dividers, pots, and a few function generator circuits in a massive collection of wires and boards going all over the place. So it can be done.
If it's an ATX power supply one has to short the power pins together to turn it on.
The major thing to be careful with is the minimum and maximum current draw. A lot of power supplies will instantly die if you turn them on with nothing connected to them. You have to either look at the specs for that particular power supply or guess (and take the associated risks). It sounds like you are using an old power supply that you really don't care about anyways.