You can but I don't know if it is safe. A DPDT is a double pole - double throw switch. A SPST is a single pole - single throw switch. Double pole is essentially two single poles controlled at the same time. Therefore, if only one side (or pole) was hooked up you would have a SPDT switch (single pole - double throw). Now, the difference between a Double throw and a Single throw is that a single throw is either on or off. Think of a light switch. Up and it is connected to the main power. Down and it isn't. A Double throw has two eligible power sources. All the way up and it would be connected to the upper power, down and connected to the lower power. These are usaully used in generator setups so that the house cannot be powered by the main power grid and the generator at the sametime(protects your appliances and the man on the pole trying to fix any electrical problems.)
So, if you hooked up your switch source to the through connection on the DPDT switch and your hot to one pole of the DPDT switch, you would essentially have a SPST. If you switch the DPDT to the side that doesn't have anything connected to it, it would be in the off position.
Once again, I do not know if this is legal or safe. I would contact an electrician.