system preferences>keyboard & mouse>keyboard shortcuts
click the plus button. change all applications to pages. type the exact name of the full screen option below that, then below that enter the shortcut key you want. restart pages. if that doesn't work try restarting your computer. some applications ignore this and i don't have iwork, so i don't know if this will work. if it doesn't work nothing happens, the keyboard shortcut just doesn't change. then you can go back to keyboard shortcuts and click minus. also because i don't have iwork i don't know if theres a built in keyboard shortcuts item like in photoshop cs4. if so, search help for the menu item.