Depends on the method you use to call the asp.net page.
If you use the POST method (e.g. form post) then you can use the standard Http Adapter, with a map and flat file assembler to construct the form message.
If you use the GET method, then you will need to write a custom request/response Http adapter.
Or you can write a .Net class to call the asp.net page and return the result. You can call the .NET class from the orchestration. If you use a static method on this class, you do not have to instantiate the class or worry about making it serializable.
The custom adapter is more complicated, but you have advantages like, using a pipeline for message manipulaton and the in-built retry capabilities.