I don't now anything about doing it but look at the main site:
http://www.s2member.com/"s2Member Pro is an add-on module for the free version of s2Member. s2Member Pro adds PayPal® Pro integration, PayPal® Pro Form Generators, Authorize.Net® AIM/ARB integration ( also w/ Pro Forms ), AliPay® ( China ), Google® Checkout integration, ClickBank® integration, ccBill® ( for adult-oriented sites ), advanced User Import/Export tools, Coupon Codes, a Pro Login Widget ( plus Profile Summary ) and more."
You might also want to try a search for "Other Payment Gateways".
DP