How extend shortcode default values for a plugin?

You can hijack the shortcode and use your own additional logic.

Sample code, not tested:

add_action( 'after_setup_theme', function()
{
    // Replace the plugin shortcode handler with your own.
    add_shortcode( 'standout-css3-button', function( $atts, $content = NULL )
    {
        // Let the plugin make the button and store the result.
        $button = scss3button( $atts, $content );

        // No custom class, stop.
        if ( empty ( $atts['cssclass'] ) )
            return $button;

        // Inject your own class, and add a white space as class separator.
        return str_replace(
            'class="',
            'class="' . $atts['cssclass'] . ' ',
            $button
        );
    });
});

By the way: I wouldn’t use that plugin on any of my sites. The code makes me cringe. You should really consider implementing the buttons you need in your theme and get away with less and better code.