add new attributes into existing shortcodes

All you need to do is modify the $a-variable, for example, to add a target to this link, you would add it to the array making up the $a-var, and printing it later.

add_shortcode( 'button', 'btn_shortcode' );
function btn_shortcode($atts, $content = null) {
    $a = shortcode_atts(array(
        'class' => 'button',
        'href'  =>  '#',
        'target' => '_blank'
    ), $atts);

    return '<a target="' . esc_attr($a['target']) . '" class="' . esc_attr($a['class']) . '" href="' . esc_attr($a['href']) . '">' . $content . '</a>';
}