Add to cart option to have multiple of 6

In the bleeding edge of WooCommerce 2.1 you can filter woocommerce_quantity_input_min and woocommerce_quantity_input_step directly. That should get you 6,12,18, etc. I have no idea if these filters are in WC2.0.?. Note the $product object is passed to the filter in case you don’t want this to be a universal change. You can get the bleeding edge of WooCommerce from github, though it probably isn’t advisable to use it in production yet.

function wpa116693_filter_min_quantity( $min, $product ){
    return 6;
}
add_filter( 'woocommerce_quantity_input_min', 'wpa116693_filter_min_quantity', 10, 2 );

function wpa116693_filter_quantity_step( $step, $product ){
    return 6;
}
add_filter( 'woocommerce_quantity_input_step', 'wpa116693_filter_quantity_step', 10, 2 );