How do I redirect a web request that does not exist in the UCM to another URL outside of the UCM

 We are in the process of moving our websites into UCM and we still have many web pages outside of the UCM that need to be integrated into the UCM and redirected outside of the UCM if the URL does not exist. Here is the scenario:

Our Web Domain name is directed to the UCM. If a previous "aspx" page does not exist in the UCM, we would like to redirect it to the URL where the "aspx" page exists. I would do this by getting the requesting URL and looking in a list of valid reroutable URLs. If the URL is found in the list, the request would get rerouted to another web server.