Windows recognizes from F1 to F24. If your keyboard goes as high as F12, you can define hotkeys that generate F13 to F24 and Windows will pass them along to WoW. You can't generate anything higher than F24 because Windows doesn't have an internal code that represents such keys. I imagine you can use any hotkey program for this purpose. My own program, HotkeyNet, can do it.

However -- this is a big however -- I don't know if WoW will respond to the additional function keys. It may be programmed to respond only to keys that are commonly found on keyboards.

By the way, there are more keys you can use like this. Keys that exist in Windows but aren't found on the typical keyboard. Here's a list of all all the keys recognized by Windows:

http://www.hotkeynet.com/ref/keynames.html

(There are no standard names for keys. Each program makes up its own names. But they represent numbers called virtual key (VK) codes which are standard. Here's a list of the VK codes: http://msdn2.microsoft.com/en-us/library/ms645540.aspx.)