Add colors to existing color palette without replacing it

You can merge palettes

$existing = get_theme_support( 'editor-color-palette' );

$new = array_merge( $existing[0], array(
    array(
        'name' => __( 'Strong magenta', 'themeLangDomain' ),
        'slug' => 'strong-magenta',
        'color' => '#a156b4',
    ),
    array(
        'name' => __( 'Light grayish magenta', 'themeLangDomain' ),
        'slug' => 'light-grayish-magenta',
        'color' => '#d0a5db',
    ),
));

add_theme_support( 'editor-color-palette',  $new);