E-junkie already uses PayPal's IPN to notify our system when a buyer's payment has completed, and any manual IPN settings in your PayPal account would be overridden by the IPN specs that our buttons and cart transmit to PayPal with every checkout, so it is not possible to use E-junkie with PayPal but have them send their IPN to anyplace other than E-junkie.
Our Integration feature is designed to mimic a PayPal IPN in every way possible, so you should be able to use that if you can figure out how to disable the function in your membership script that would verify the IPN with PayPal (which would fail, since we are not PayPal); you may need to contact your script's developer to find out how to do that.