Extracting an interface from the implementation is not going to help you much anyway, as it's going to be a poor abstraction.
Interfaces should be defined and owned by the clients that consume the interfaces. As Agile Principles, Patterns, and Practices explain, "clients […] own the abstract interfaces" (chapter 11). Thus, any attempt to define a data-centric interface like extracting interfaces from auto-generated web service clients is bound to cause problems sooner or later, because it's violating various SOLID principles like the Dependency Inversion Principle or the Interface Segregation Principle.
Instead, your client code should define the interfaces that they require. Then you can always implement those interfaces with the auto-generated web service clients. If you've used Microsoft's tooling (Visual Studio, wsdl.exe, etc.) the relevant auto-generated class should already be a partial class, which means you should be able to add behaviour to it without touching the auto-generated part of it.