You can't.
In order to change the port(s) or address(es) the web server is listening on you must change the web server configuration and restart the web server.
I think your best options are:
Using mod_proxy
to pass your app through Apache
Have your app listen on some other port and configure mod_proxy appropriately
Have both Apache and your app listen on different ports and use a third application to listen on port 80 and forward the requests to the appropriate service.
Basically writing your own socket-bouncing proxy.
Writing that socket-bouncing proxy (or configuration of mod_proxy) is left as an exercise for the reader - mainly because I don't know your internal requirements.