I am running Ubuntu 13.04 in a Live (USB) session. My physical keyboard layout is called Bépo, it is the French language Dvorak method-based layout. I would like to change my input layout to this too.

To do this, I tried booting in a French spoken session, then open the Keyboard Layout preferences app. Normally, to add a layout, you should click on the + sign and select your layout. However the list that appears is very short and does not contain what I want. On the contrary, on a persistent non-live installation, the choice of Bépo is present. This is also the case during an installation of Ubuntu.

So I do I change the keyboard layout of my live session to the correct one?