X special keys as opera shortcuts

Hi,
can you define a keyboard shortcut referring to an "X" special key, for
instance XF86Back and XF86Forward ?
This is not academic because my 4th and 5th mouse buttons are mapped to
XF86Back and XF86Forward in order to use them with
konqueror and Firefox.
It would be nice if you could bind for instance XF86Mail to opera mail. It
works so on windows.
Can you use their key code as shown by xev in alternative ?
Bye,
Giacomo
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