How To Use Facebook's Conversion Tracking with CPA Offers?

I'm trying to set up a "Website Conversions" campaign on Facebook to promote a CPA offer. But in order to use a "website conversions" campaign effectively , I need to tell facebook what a conversion is. In my case, a conversion will not be one of the standard events that they offer, so I need to setup a custom conversion.

So my question is, how do I track conversions for a cpa offer?

Or even better, but less likely, is it possible to use the offer's postback as a conversion on facebook?

thanks...

Wondering the same

Still trying to figure this out..

Seems like nobody here knows.
Anyone got any suggestions on forums where i can find someone might knows how to do this?