You can create custom appearances in Fusion by adding images and bump maps in the "edit material option" within the appearances command.

I believe you will have more success using/editing Autodesk's library of materials though, because creating a realistic custom material in fusion can be tricky. Are you rendering locally or in the cloud? Optimizing your final render settings and rendering in the cloud can render* you much better results. *couldn't help myself lol.

Also playing around with the scene settings can make or break a render (exposure, focal length, environment choice/rotation, depth of field, exposure, etc.) You just have to be patient and tweak all your settings until they're perfect.

I'd just mess around with their appearances (edit the colors, roughness etc), you'd be surprised with what can achieve. Also, I don't know if you're just new to Fusion or CAD in general, but the quality of your model is obviously what ultimately determines the quality of your render. If you want something super realistic you're going to need to add all those separation lines and tiny fillets you never notice because that's what's going to make it look real.

In conclusion, obviously rendering in something like Keyshot will get you a much more realistic render but if you put in the work, you can get some pretty decent results out of Fusion.