define new shortcut keys without overwriting existing keys

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http://askubuntu.com – My laptop keyboard does not have media shortcut keys (e.g. "Next Track", etc). I frequently use the laptop connected to an external keyboard, which does. I go to Settings->Keyboard->Shortcuts tab, and select "Sound and Media", and I see that "Next Track" is bound to the shortcut key "audio next". I see that I can replace this with a custom key binding, but it overwrites the "audio next" shortcut. I'd like a solution that allows me to map a second shortcut to this command without overwriting it. I see that I can define custom keys, so I could make a unique mapping for rhythmbox-client -- (HowTos)