Dear Shrine,
You can use SOAP Extensions. A beautiful concept in .NET.

SOAP extensions enable you to access and modify the SOAP messages that are exchanged between the client and the XML Web service. For example, you can use SOAP extensions to validate the SOAP message before it reaches the XML Web service and change the SOAP message after validation so that the XML Web service receives a valid SOAP request. You can also implement an encryption or compression algorithm that can be executed within an existing XML Web service by using SOAP extensions.

It is having 4 methods...

AfterDeserialize
This member represents the stage just after a SoapMessage is deserialized from a SOAP message into an object.

AfterSerialize
This member represents the stage just after a SoapMessage is serialized but before the SOAP message is sent over the wire.

BeforeDeserialize
This member represents the stage just before a SoapMessage is deserialized from the SOAP message that is sent over the network.

BeforeSerialize
This member represents the stage just before a SoapMessage is serialized.

You just dig this, you will get more and you can achieve your task shortly.