As others have said, it isn't possible. A belt drive frame has the rear seat stay bolted to the drop out in some fashion, I've seen different varieties, so that the belt can be slipped into the frame. This is because the current chain runs from inside the frame at the hub to outside the frame at the crank both top and bottom and without the break in the frame you have to pin the chain, the belt offers no such option.
Personally, I've run nexus hubs in the past, used one for a commuter. But your 7.3fx wouldn't benefit from the conversion and in my experience that is a nice fitness/commuter bike that doesn't lack for much. Sure, there's better, but it is just a generally nice bike as is. Using an alfine or nexus you want to pair the gearing of the crank to match the ratios of the hub and which has a proper chainline from crank to cog. So even skipping the belt you're probably looking at a crankset, bb, rear hub,/wheel, new shifter, and new grips to make it all work. A lot of work for a bike that's already nice and functioning. Now buying a frameset and building a bike because you want a r+1, not always worth the cost but still a worthwhile endeavor just to see what you can accomplish.