How to extend page web services

Take a look at http://msdn.microsoft.com/en-us/library/dd338962.aspx

Then define your Object as Parameter of the function.

The first parameter of the code unit function that extends a page should be the object that the page is based on (Customer). Your clients first parameter for that extended function is the key that you obtained when invoking the read operation on the page based web service. The web service will automatically give the object to your code unit. If you need to pass additional objects as parameters, I would pass record Id's.