That depends on what you'd call another web app.
If you want to redirect to another URL with a Button, just add a link widget into the mashup, hide it, and bind the click event of the button to navigate service of link widget.
Or you can create a new responsive mashup, put the web frame widget into it and call that mashup with navigation widget which is more or less a button.
Be careful though, since web frame widget is basically HTML iframe, some sites will not work in it. Hope I helped.