NetBeans engineers are working on better Java -> Java FX integration. You should be able to build NetBeans UI modules with Java FX and integrate them so with a Swing app. NetBeans templates and build process will be also enhanced to seamlessly compile the module.
The Java FX code is referenced in the XML-file - there is no difference to the "old" Java approach here.
A suggestion: it should be possible to use an adapter to load a Java FX (Folder) class with javafx.util.FXEvaluator at runtime instead the XML-file - it has already a declarative syntax.
Then you would be able to build the entire app with Java FX - without any XML. It would be also type safe.