Help me extend marketpress

I'm a developer and I'd like to extend marketpress to take recurring payments like membership and / or pro-sites. (The goal is to incorporate the pro sites level payment into a shopping cart and combine with other purchases) please give me an idea where in the membership or pro sites code I can look for an example of how the recurring payments are handled. I thought another approach could be to call a pro sites method to set up the account when triggered by successful checkout in marketpress. Please advise. Thanks!!