Modern UI is not exclusive to WinRT.
No, Modern UI is exclusive to WinRT, unless you're talking about skinning a desktop application to look like a Modern UI application. But it won't be able to integrate with all the WinRT services or run in a metro frame (full-screen independent of the desktop).
There is an exception to this rule, the default web browser. Web browsers are the only apps (that I know of) that can hook into some of the Modern UI capabilities without having to be built for WinRT. You can also download them without going through the Windows Store. This is how Chrome is allowed to have a Modern UI mode. But it requires the application to be running as the default web browser, meaning it's not a feasible workaround for doing the same with other applications.