I don't know any way (or reason) to use JavaFX and Java2D at the same time in the same stage/window. JavaFX has ways to running Swing controls, I think. I really haven't looked closely into this.
If you want an easy intro to JavaFX, this Java-Gaming tutorial
could be helpful.
Otherwise, I recall that others have made particle systems and posted code here before, a few years back.
I tried a search on "particle system Java2D" in the local search box. Several threads came up. Maybe StumpyStrust's tutorial
will be a help get you what you want.