Well, that's the problem. I don't have a generic button on my keyboard and I don't know if I can map it! I need it to create a new key, like on a gamepad. I'm thinking I could take an uncommon key (bad position) and map that to caps....
You could do what I've done for my Logitech keyboard. I never used the numberpad for in-game binds, so I mapped one of the G keys (in your case Capslock) to KP_9.
Then ingame I bound whatever function I wanted to that G key, which now registers as KP_9.
Works like a charm, especially if you don't use that side of the keyboard much/at all.
The benefit of this is that the keys aren't used in the first place for default game mappings, and it has no adverse effect to applications outside the game either.
Keyboards with mappable keys require their own software to map to multiple key strokes / macros, and can only map to existing keys.