Adding custom resolutions is easy. As you found out, the WS resolution you attempt to use in games needs to be a resolution that you can set your desktop to.
If you have a Nvidia card, just go to the settings for the card (either through Display-> Advanced->Your Card Model, or if its enabled just right-click the desktop and click your monitor name or whatever.
If you have an ATI, unless things have changed since I upgraded my 9800 Pro, you'll need a 3rd party program to add custom resolutions. Powerstrip is one of them. Just load it up and look for custom resolutions, and add the resolution.
Make sure its a valid 4:3, 16:10, etc resolution. Some games only have support for 16:9 resolutions for whatever reason (there's pretty much no PC monitors that are 16:9, and a very small % of people use a LCD TV as a computer display) but I'll take a slightly stretched 16:9 vs. no WS or a stretched 4:3. If you have an LCD keep it safe and set it to 60 Hz refresh rate.