So you want to create a web service method, and the return value of the web service method is based on a another web services out there (i.e: http://mydomain.aspx/WebService.asmx?12334)?
You didn't indicated how much you know about web services, so I'm just gonna outline some key points here:
1) open vs.net pick your favor language, and create a "web service" application
2) you should see <web service()> Hello method, take that off and replace with whatever name you prefered
3) expand References icon under Project.
4) click "add web references"
5) copy and paste your URL (i.e: http://mydomain.aspx/WebService.asmx?12334)
6) the box in your left will shows the avaiable methods, at this point, i assume you should know which method is doing what....
7) create your proxy object based on the url, and call the correct method to extra the information you are looking for
8) do whatever operation as needed, then return value (or object)
9) [this is the part i hate the most...], write documentation, so your user can use your web services