Thank you for your inquiry.
But the link provided by you says that you guys don’t provide any support for PAYMILL.
Can I make it by customizing the s2members code by some developer?
Yes, you could integrate s2Member with Paymill with custom code. However, as per our Support Policy, we do NOT offer professional installation service, customizations, or services involving custom coding. If you need assistance in this regard, we recommend http://jobs.wordpress.net, or another freelance web site where WordPress® experts are offering their expertise through a bid on your project.
See: s2Member® » Support Policy
s2Member does provide some tools for making integration with another payment gateway a bit easier. To get started, I would recommend checking out the information on s2Member’s PayPal IPN with Proxy Key.
See: Dashboard -› s2Member® -› PayPal® Options -› PayPal® IPN Integration -› IPN w/ Proxy Key ( optional, for 3rd-party integrations )
The IPN w/ Proxy Key functionality allows a developer familiar with Paymill and s2Member use the information provided from Paymill, and send it to s2Member to process as though it were sent directly from PayPal.
As of s2Member® Pro v130203
s2Member® Pro v130203 was updated to support some additional Remote Operations (create_user, modify_user, delete_user). So aside from s2Member’s PayPal IPN Proxy (which is still the best way to integrate a custom payment gateway), the s2Member® Pro Remote Operations API is now a viable alternative.
See: Dashboard -› s2Member® -› API / Scripting -› Remote Operations API
See Also: Dashboard -› s2Member® -› PayPal Options -› IPN Integration -› More Information -› IPN Proxy Key
See Also: s2Member -› Changelog v130203
Keep in mind that this API is designed to perform these specific tasks. It is NOT designed to handle post-processing of transactions and trigger API Notifications, emails, etc. If you integrate a custom payment gateway and you want to maximize compatility with everything that s2Member® offers, use the PayPal IPN Proxy provided by s2Member.
If you want to set things up in a truly custom way (handle your own events, emails, etc.), and perhaps simplify the integration a bit, go with the Remote Operations API instead.
Let us know if you have any further questions/concerns. :-)