Right now you have only the 2 renderer that fusion comes with (software and OpenGL)
But you can implement other render engines through the sdk. The same as in any 3d app.
A long time ago I created a proof of concept for 3delight (based upon the great example for renderman in the sdk)
You can still the WIP from the project :
https://reichofevil.wordpress.comIf enough people ask a specific renderer developer/company they will see a market and maybe provide a plugin to render with