No, you can not add a web reference at runtime. Adding a web reference creates several classes that allow you to interface with the web service; those classes need to be compiled before they can be used.
If you want to dynamically support different types of web services, they'll have to use an consistent interface so that a single set of classes can use the web services. Otherwise, you'll have to implement something that queries the web service's definition and creates some sort of user interface for the user to enter the data that needs to get to the web service.
Please go here you can get more some information
http://social.msdn.microsoft.com/Forums/vstudio/en-US/39138d08-aa08-4c0c-9a58-0eb81a672f54/adding-a-web-reference-dynamically-at-runtime[
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