Remap Shift + space to underscore_ with Karabiner

private.xml

<?xml version="1.0"?>
<root>
    <item>
        <name>shift space</name>
        <identifier>private_shift_space</identifier>
        <autogen>
            __KeyToKey__
            KeyCode::SPACE, ModifierFlag::SHIFT_L,
            KeyCode::MINUS
        </autogen>
        <autogen>
            __KeyToKey__
            KeyCode::SPACE,  ModifierFlag::SHIFT_R,
            KeyCode::MINUS,  ModifierFlag::SHIFT_R
        </autogen>
    </item>
    <item>
        <name>PC useless keys for volume</name>
        <identifier>private_volume_keys</identifier>
        <autogen>
            __KeyToKey__
            KeyCode::PC_PRINTSCREEN,
            ConsumerKeyCode::VOLUME_MUTE
        </autogen>
        <autogen>
            __KeyToKey__
            KeyCode::PC_SCROLLLOCK,
            ConsumerKeyCode::VOLUME_DOWN
        </autogen>
        <autogen>
            __KeyToKey__
            KeyCode::PC_PAUSE,
            ConsumerKeyCode::VOLUME_UP
        </autogen>
    </item>
</root>

Left shift + Space -> - (minus)
Right shift + Space -> _ (underscore)

Print screen -> Volume Mute
Scroll lock -> Volume down
Pause -> Volume up