Is it possible? There was a mod that enabled you to switch songs from inside World of Warcraft, but it no longer works. And I seem to recall that the Windows Task Manager used to come up on top of full screen games, so I have to imagine that games aren’t entirely locking off the screen.
I don’t know how you would do that unless there is a “Keep on top while playing” option somewhere in the settings. Failing that you can hold down alt and press tab, this will allow you to quickly switch between itunes and WoW quickly and easily.
The miniplayer does have a “keep on top” option, but playing the game full screen trumps that. I think your only option is to play WoW in windowed mode, if that’s possible.