Sunil,
What I would do in your case is create a new Webservice. This webservice
will have 2 types of methods in it:
1) Methods that are wrappers to the webservice you cannot control. These
will have the same parameters as the original method. When invoked, they
will call the original method (on the service you do not control) and return
that services results to the caller.
2) The enhanced methods, which are these new method you are trying to add.

To make things easier, you could just provide 2 webservices also. The
original, and a new one with your enhanced functionality.

HTH
S