That being said, PayPal's service "Website Payments Standard" will not allow the IPN communication to be transferred from your old system to your new system. At least not for existing Subscriptions that existed before you made the switch to s2Member. Going forward you'll be fine, but old "Subscriptions" will NOT be communicated to s2Member via IPN, because they were created by a different piece of software
I use an IPN Relay script to receive the PayPal "Website Payments Standard" IPN and then forward it to three different scripts, WishList Member, iDevAffiliate, and a custom script I use to subscribe new members to my autoresponder. Each uses the IPN properly, extracting the information it needs. The only data in the CUSTOM field is the buyers IP address, which iDevAffiliate uses in some way.
If I use this IPN relay script to also point the IPN to the correct S2 URL, will S2 read the IPN and manage subscribers I have manually added?