Possible to add new key combinations to keyboard shortcuts?

Under Tools > Customise... > Keyboard you can modify current “hotkey” assignments, or add new ones to the list of available combinations:

I would like to use CTRL ALT - as a key combo, but the only + or - shortcuts available in this dialog are those highlighted in the screenshot above.

I don’t see a way of adding key combinations, just modifying this listing. So – is there is a way to add new key combinations to this list? I live in hope!

(Using LibO under Ubuntu 14.04, fwiw!)