I use “Add Link” quite a lot and wonder if there is some hidden keyboard shortcut to access it…
There isn’t, but you can add one yourself using a great feature of OS X that not many people know about. Go to your System Preferences and choose “Keyboard and Mouse” (or just “Keyboard” on Snow Leopard). The “Keyboard Shortcuts” pane allows you to specify a keyboard shortcut for any menu item in any application. Just make sure it doesn’t conflict with an existing shortcut.
All the best,