The Cinema 6 comes with 4 "blank" buttons to learn extra functions onto.

Although the book doesn't say so, you can also learn a function onto most of the other keys, replacing whatever pre-programmed function was on the key originally. The procedure is exactly the same, simply follow the instructions but press the key you want instead of one of the L keys.

Just remember that the storage for learned functions is limited to about 30 keys total across all device modes. (The exact number varies depending on what code(s) you are learning.) If your remote starts refusing to learn more keys, it is probably full.