Paid subscription - Paypal refferal ID for affiliate?

[Kraklin]

I run a paid only membership forum.

I currently have it set up so users register first, then have to go into their settings to upgrade to a paid member through paypal.

Is there a way to allow affiliates to send over traffic, and once someone they referred signs up I can see who referred them? (they aren't members of my forum tho) like a URL refferal ID.. or a way I can add a section in the paypal form for people to insert who referred them?

I sent this question to paypal and this is the response I got:

PayPal generates a unique PayPal ID number for your subscribers.

* This PayPal ID number is shown in email notifications, Instant
Payment Notifications, and your Downloadable History Log.
* You can also track subscribers using item numbers, invoice
numbers, and custom numbers.

Is there a way I can insert this paypal ID number to my IPN so I can see all my current paid subscribers and which ones were brought by a certain URL or ID.