@istota:
In principle it should be sufficient to load another keyboard layout. If you have a look at the rx-51 configuration file you'll find that modifying it is pretty simple (nevertheless make a backup!). Just create a new "keyboard layout" or change an existing one to match your USB keyboard. After doing that getting your USB keyboard to work properly in Easy Debian should be as simple as changing the keyboard layout (in Easy Debian, not in Maemo).
However this is only theoretical since I've never tried it. I'm not sure if the different geometry (e.g. number of key rows) would cause any trouble.