Hi,
Since we don't have the documentation for WorldPay, nor a test merchant account, and that we didn't develop the plugin ourselves, it will be hard for us to add that. That plugin was first provided by a third party developer, and we're just maintaining it so that it continues to work with new versions of HikaShop.
So adding that could be great, but we would need more information, or even better, if you would provide the code you added to make it work like you want, we could reintegrate them in the default plugin.