You'll need to build that into your app. For example, you could have them enter the payer and subscriber details in a form prior to sending them over to PayPal. Save those details in your database and then pass the record ID over to PayPal along with the payment details. You could use the invoice parameter for the custom parameter for this, for example.
That value would then come back in IPN so you could easily query your database to pull all the details back out and process things accordingly.