I know a little bit about WCF :) but never really used it in anger in BizTalk and also did not get an opportunity to extend the adapter.
I was looking for something else and came across this post form Paolo which explains in a lot of detail how does one go about extending and customizing WCF adapters. If you don’t know anything about WCF the first part explains that in general before going in to the BizTalk specific things.