Multiple Paypal IPN

I know Pro sites has a central IPN forwarder for other Pro sites and wpmuDev plugins But I do need this for a Pro Site I would Like to setup and another php Site that uses IPN
at the moment its setup for the php Site.
I know it can be done but I would need to modify the php site.
Does anyone know if it can be done with out modifying the php site and or Better yet Software That I can install on a domain that would take care of Both Sites? Pay or Free is fine